RBSE Solutions Class 10 Science Chapter 11 Work, Energy and Power

I. Multiple Choice Questions

Question 1: What is the unit of work?
(a) Newton
(b) Joule
(c) Watt
(d) None of the options
Answer: (b) Joule
In simple words: The unit for measuring work is called a Joule. It is a standard way to express how much energy is transferred when a force makes something move.

🎯 Exam Tip: Remember that Joule is also the unit for energy. Work and energy are closely related concepts, often used interchangeably in terms of units.

Question 2: If angle between force F and displacement s is \( \theta \) then work done is which of the following?
(a) \( F s \sin\theta \)
(b) \( F s \)
(c) \( F s \cos\theta \)
(d) \( F s \tan\theta \)
Answer: (c) \( F s \cos\theta \)
In simple words: When a force pushes an object over a distance, the work done depends on the angle between the push and the direction of movement. If the force and movement are in the same direction, work is maximum.

🎯 Exam Tip: The formula for work done involves the cosine of the angle because only the component of force acting in the direction of displacement contributes to the work.

Question 3: An object of mass m is moving with velocity y. What is the value of kinetic energy?
(a) \( mv \)
(b) \( mgv \)
(c) \( mv^{2} \)
(d) \( \frac{1}{2} mv^{2} \)
Answer: (d) \( \frac{1}{2} mv^{2} \)
In simple words: Kinetic energy is the energy an object has because it is moving. Its value depends on how heavy the object is (mass) and how fast it is going (velocity), specifically the square of the velocity.

🎯 Exam Tip: The `y` in the question refers to velocity (`v`). Always remember that kinetic energy has a squared term for velocity, meaning that doubling the speed quadruples the kinetic energy.

Question 5: What is the unit of power?
(a) Newton
(b) Watt
(c) Joule
(d) Newton meter
Answer: (b) Watt
In simple words: Power is how fast work is done or how quickly energy is used. The unit for power is called a Watt.

🎯 Exam Tip: Differentiate between work (Joule) and power (Watt); power is work done per unit time. One Watt equals one Joule per second.

Question 6: How much work is done in moving an object of mass 1 kg to a height of 4 m? (g. 10 m/s\(^{2}\))
(a) 1 Joule
(b) 4 Joule
(c) 20 Joule
(d) 40 Joule
Answer: (d) 40 Joule
In simple words: To lift an object, you do work against gravity. You can calculate this work by multiplying the object's mass by gravity and the height it is lifted.

🎯 Exam Tip: Work done against gravity (to lift an object) is equal to the change in its potential energy, calculated as `mgh` (mass x gravitational acceleration x height).

Question 7: What happens to total energy of a free falling object?
(a) Keeps on increasing
(b) Keeps on decreasing
(c) Remains constant
(d) Becomes zero
Answer: (c) Remains constant
In simple words: When an object falls, its energy changes from potential energy (due to height) to kinetic energy (due to motion), but the total amount of energy stays the same. This is the law of conservation of energy.

🎯 Exam Tip: The law of conservation of energy applies to isolated systems, meaning no external forces are doing work on the object, like air resistance.

Question 9: What is the commercial unit of electric energy?
(a) Joule
(b) Watt second
(c) Kilowatt hour
(d) Kilowatt per hour
Answer: (c) Kilowatt hour
In simple words: When we talk about how much electricity homes use, we use a unit called a kilowatt-hour. This is how electricity bills are calculated, not in Joules.

🎯 Exam Tip: Remember that 1 kilowatt-hour (kWh) is the energy consumed by a 1-kilowatt appliance running for 1 hour, which equals \( 3.6 \times 10^6 \) Joules.

Question 10: If spring constant is k, then what happens to its potential energy on compressing the spring to distance x within its elastic limit?
(a) \( kx \)
(b) \( \frac{1}{2} kx^{2} \)
(c) \( kx^{2} \)
(d) None of the options
Answer: (b) \( \frac{1}{kx^{2}} \)
In simple words: When a spring is squeezed or stretched, it stores potential energy. This stored energy increases more quickly as the spring is compressed further.

🎯 Exam Tip: Be careful with the square term for displacement (x) in spring potential energy calculations, as small changes in compression lead to larger changes in stored energy.

Work, Energy and Power Very Short Answer Type Questions

Question 1: Define work and write its unit.
Answer: Work is done when a force causes an object to move a certain distance in the direction of the force. The unit of work is the Joule (J). It can also be expressed as Newton-meter (Nm).
In simple words: Work means moving something using a force. Its unit is a Joule.

🎯 Exam Tip: Clearly state both the definition (force causing displacement) and the unit (Joule or Newton-meter) for a complete answer.

Question 2: What is the unit of energy?
Answer: The unit of energy is Joule (J). Energy can be found in many forms, like heat, light, sound, and movement.
In simple words: Energy is measured in Joules.

🎯 Exam Tip: The unit for energy is the same as the unit for work, as energy is the capacity to do work.

Question 3: What is kinetic energy?
Answer: Kinetic energy is the energy an object possesses due to its motion. Moving objects have kinetic energy.
In simple words: Kinetic energy is the energy that things have when they are moving.

🎯 Exam Tip: To score full marks, define kinetic energy clearly and state that it's related to movement, not position.

Question 4: What is potential energy?
Answer: Potential energy is the energy stored in an object because of its position or state. For example, an object held at a height has gravitational potential energy.
In simple words: Potential energy is stored energy in an object due to where it is or how it is arranged.

🎯 Exam Tip: Remember that potential energy depends on position (like height or a stretched spring) while kinetic energy depends on motion.

Question 5: Write the law of conservation of energy.
Answer: The law of conservation of energy states that energy cannot be created or destroyed. It can only be transformed from one form to another, but the total amount of energy in a closed system always remains constant.
In simple words: Energy never disappears or appears from nowhere; it just changes from one type to another, keeping the total amount the same.

🎯 Exam Tip: When writing about conservation laws, always mention that energy is transformed, not created or destroyed.

Question 6: Dissipation of energy generally happens in which forms?
Answer: Energy dissipation typically occurs in the forms of heat, light, and sound. These are often considered "lost" energy because they are not useful for the intended purpose of the energy conversion.
In simple words: When energy changes forms, some of it often turns into heat, light, or sound and can't be used for what we want.

🎯 Exam Tip: Always list common forms of dissipated energy, like heat, sound, and light, as they are indicators of inefficiency in a system.

Question 7: Is it possible to make a hundred percent efficient system in terms of energy?
Answer: No, it is not possible to create a system that is 100% efficient in terms of energy. Some energy will always be dissipated, usually as heat, due to friction or other factors.
In simple words: No, we cannot make a machine that uses all its energy perfectly without losing any.

🎯 Exam Tip: Understand that the second law of thermodynamics implies that 100% energy efficiency is impossible in real-world systems.

Question 8: What do you understand by electric energy?
Answer: Electric energy is the energy possessed by charged particles when they are in motion or at a certain position relative to other charges. It is crucial for powering all our electronic devices.
In simple words: Electric energy is the power that comes from tiny charged particles, like what runs our gadgets.

🎯 Exam Tip: Relate electric energy to the movement or position of charged particles to fully define it.

Question 9: Write the name of any three electrical appliances.
Answer: Three common electrical appliances are an electric fan, a light bulb, and a refrigerator. These devices transform electrical energy into other useful forms.
In simple words: An electric fan, a light bulb, and a refrigerator are three examples of things that use electricity.

🎯 Exam Tip: When asked for examples, choose common, easily recognizable items that clearly demonstrate the use of electrical energy.

Question 11: Which type of light is suitable for reducing electricity consumption in homes?
Answer: LED (Light Emitting Diode) and CFL (Compact Fluorescent Lamp) lights are suitable for reducing electricity consumption in homes. They use less power to produce the same amount of light compared to traditional incandescent bulbs.
In simple words: LED and CFL bulbs help save electricity at home because they use less power.

🎯 Exam Tip: Always mention both LED and CFL when discussing energy-efficient lighting, as they are key examples of modern lighting technology.

Question 12: What should be kept in mind while buying a new electrical appliance?
Answer: When buying a new electrical appliance, one should keep its Star Rating in mind. A higher star rating indicates greater energy efficiency and lower electricity consumption.
In simple words: When buying new electric items, always look for the Star Rating; more stars mean it uses less electricity.

🎯 Exam Tip: The Star Rating is a quick and effective indicator of an appliance's energy efficiency, helping consumers make environmentally and economically sound choices.

Question 13: An object is displaced by 10 m when a force of 20 N is applied on it. Find the work done.
Answer: To find the work done, we use the formula Work = Force \( \times \) Displacement. Given force \( F = 20 \,N \) and displacement \( s = 10 \,m \).
So, Work Done \( = 20 \,N \times 10 \,m = 200 \,Nm \). The unit Newton-meter is also known as Joule (J).
In simple words: If you push something with 20 Newtons of force for 10 meters, you do 200 Joules of work.

🎯 Exam Tip: Ensure that the force and displacement are in the same direction for the simple Work = Force x Displacement formula to be directly applicable.

Question 14: It takes 1 minute to lift an object of 30 kg mass by 2 m. How much energy is spent in doing this work?
Answer: To lift the object, the work done is against gravity. We are given mass \( = 30 \,kg \), height \( = 2 \,m \), time \( = 1 \,min = 60 \,s \), and gravitational acceleration \( g = 10 \,m/s^{2} \).
The energy spent to lift the object is equal to the potential energy gained: Energy \( = mgh = 30 \,kg \times 10 \,m/s^{2} \times 2 \,m = 600 \,J \).
The power (rate of energy spent) is calculated as: Power \( = \frac{Energy}{time} = \frac{600 \,J}{60 \,s} = 10 \,W \).
In simple words: To lift a 30 kg object up by 2 meters, 600 Joules of energy is used. If this takes 1 minute, the power used is 10 Watts.

🎯 Exam Tip: Be careful to distinguish between energy (measured in Joules) and power (measured in Watts), which is the rate at which energy is used or work is done.

Question 15: If a 60 W bulb is used for 8 hours per day then how many units of electricity is consumed in 30 days?
Answer: Given the power of the bulb is \( 60 \,W \). It is used for \( 8 \) hours per day for \( 30 \) days.
First, convert power to kilowatts: \( 60 \,W = 0.06 \,kW \).
Total time of use \( = 8 \text{ hours/day} \times 30 \text{ days} = 240 \text{ hours} \).
Energy consumed \( = \text{Power} \times \text{Total time} = 0.06 \,kW \times 240 \,h = 14.4 \,kWh \).
Since \( 1 \text{ unit} = 1 \,kWh \), the total electricity consumed is \( 14.4 \) units.
In simple words: If a 60 Watt bulb is on for 8 hours daily for 30 days, it will use 14.4 units of electricity.

🎯 Exam Tip: Always convert power to kilowatts and time to hours when calculating electricity consumption in kilowatt-hours (units).

Question 2: An object is moving with velocity u. Its velocity becomes v when force F is applied on it. If the object covers a distance s during this then calculate the increase in kinetic energy of object.
Answer: To calculate the increase in kinetic energy, we can use the work-energy theorem. According to Newton's third equation of motion, we have:
\( v^2 = u^2 + 2as \)
We can rearrange this to find acceleration \( a \):
\( a = \frac{v^2 - u^2}{2s} \)
From Newton's Second Law of Motion, Force \( F = ma \). Substitute the value of \( a \):
\( F = m \left( \frac{v^2 - u^2}{2s} \right) \)
Now, work done \( W = F \times s \). Substitute the value of \( F \):
\( W = m \left( \frac{v^2 - u^2}{2s} \right) \times s \)
\( W = \frac{1}{2} m (v^2 - u^2) \)
This work done is equal to the change in kinetic energy:
\( W = \frac{1}{2} mv^2 - \frac{1}{2} mu^2 \)
This equation shows that the work done on the object is equal to the change in its kinetic energy.
In simple words: When a force makes an object speed up, the work done by that force equals how much the object's movement energy (kinetic energy) increases. We use motion equations to show this connection.

🎯 Exam Tip: This derivation highlights the work-energy theorem, which states that the net work done on an object equals the change in its kinetic energy. Mastering this concept is crucial for solving many physics problems.

Question 3: What is potential energy? If an ideal spring has spring constant k then find the acquired potential energy in compressing the spring by distance x.
Answer: Potential energy is the energy an object possesses because of its position or configuration. For a spring, if the spring constant is \( k \) and it is compressed by a distance \( x \), the force applied can be given as \( F = kx \).
The work done in compressing the spring by distance \( x \) is stored as its potential energy. Work done is calculated by integrating force over displacement. For a spring, the average force is \( \frac{1}{2} F_{max} = \frac{1}{2} kx \).
So, Work Done \( = \text{Average Force} \times \text{Displacement} = \frac{1}{2} kx \times x = \frac{1}{2} kx^{2} \).
Thus, the potential energy (PE) acquired by the compressed spring is:
\( PE = \frac{1}{2} kx^{2} \). This energy is released when the spring returns to its original shape.
In simple words: Potential energy is stored energy from an object's position. For a spring, when you squeeze it, it stores energy equal to half of its spring constant times the square of how much you squeezed it.

🎯 Exam Tip: Remember the formula \( PE = \frac{1}{2} kx^2 \) for a spring's potential energy. It's a common application of work done by a variable force.

Question 4: An object is moving with velocity v. If mass of the object is m, then how much work needs to be done to bring this object to rest?
Answer: When an object is moving with a certain velocity, it possesses kinetic energy, given by the formula \( KE = \frac{1}{2} mv^{2} \). To bring this object to rest, an external force must do work on it, specifically negative work, to reduce its kinetic energy to zero. According to the work-energy theorem, the work done to stop the object is equal to the initial kinetic energy it had. Therefore, the amount of work that needs to be done to bring the object to rest is \( \frac{1}{2} mv^{2} \). This work is done in the opposite direction of the object's motion.
In simple words: To stop a moving object, you need to apply work equal to the energy it has from moving (kinetic energy). This work will be \( \frac{1}{2} mv^{2} \).

🎯 Exam Tip: The work-energy theorem is fundamental here: the work done to change an object's speed is exactly equal to the change in its kinetic energy. To stop an object, the work done must be equal to its initial kinetic energy.

Question 6: When an object is in free fall then its potential energy is decreasing continuously. How is mechanical energy being conserved in this situation?
Answer: Let's consider a stone held at a certain height. At this height, the stone has maximum potential energy and zero kinetic energy. When the stone is released and begins to fall, its height decreases, causing its potential energy to continuously decrease. Simultaneously, as it falls, its speed increases, causing its kinetic energy to continuously increase. The law of conservation of mechanical energy states that the sum of potential and kinetic energy (mechanical energy) remains constant if only conservative forces (like gravity) are acting. So, the decrease in potential energy is exactly balanced by the increase in kinetic energy, keeping the total mechanical energy constant throughout the fall. When the stone hits the ground, its potential energy becomes zero, and its kinetic energy reaches its maximum value.
In simple words: As a stone falls, its height energy (potential energy) goes down, but its movement energy (kinetic energy) goes up by the same amount. So, the total energy it has always stays the same.

🎯 Exam Tip: For conservation of mechanical energy problems, always remember to show that the sum of potential energy and kinetic energy at any point remains constant, assuming no air resistance.

Question 7: How is energy dissipated?
Answer: Energy dissipation occurs during the conversion of energy from one form to another. In any real-world process, it's impossible to convert all energy into the desired useful form; some of it is always 'lost' or dissipated into less useful forms. For example, when a ceiling fan is switched on, electrical energy is mainly converted into kinetic energy to make the blades rotate. However, some electrical energy is also converted into heat due to resistance in the motor and friction, making the fan motor warm, and some into sound energy. This heat and sound are dissipated into the surroundings and are not used for the primary function of cooling.
In simple words: When energy changes from one type to another, some of it always spreads out as heat, light, or sound and cannot be used for the main task.

🎯 Exam Tip: Focus on explaining that energy dissipation means energy converting into forms that are not useful for the intended purpose, often as heat due to friction or resistance.

Question 8: Explain dissipation of energy from production of electricity to transmission of electricity to households.
Answer: Energy dissipation occurs at multiple stages from electricity production to its transmission to homes. In a thermal power plant, for instance, fuel is burned to heat water and create steam. This process involves significant heat loss to the surroundings, meaning not all chemical energy in the fuel converts to heat. The steam then drives turbines, converting thermal energy into kinetic energy, and then generators convert kinetic energy into electrical energy. During these conversions, some energy is always lost as heat due to friction in moving parts and inefficiencies in the generators. For transmission, electricity travels through long power lines. These lines have electrical resistance, which causes a portion of the electrical energy to be converted into heat and dissipated into the air, especially over long distances. So, less electrical energy reaches households than what was initially generated.
In simple words: Energy is lost as heat at every step from making electricity to sending it to homes, like when fuel burns or electricity travels through long wires.

🎯 Exam Tip: When explaining energy dissipation, identify specific points of loss (e.g., combustion, friction in turbines, resistance in wires) and the forms of energy lost (e.g., heat, sound).

Question 9: How are work, energy and power related to each other?
Answer: Work, energy, and power are interconnected concepts in physics. Energy is defined as the capacity or ability to do work; without energy, no work can be performed. Work, in turn, is the process of transferring energy, where a force causes displacement. Power is the rate at which work is done or the rate at which energy is transferred. It indicates how quickly work can be completed. Therefore, work requires energy, and power determines the speed of that work and energy transfer, linking all three concepts intrinsically.
In simple words: Energy is the ability to do work, work is done when energy is moved, and power is how fast that work and energy transfer happens.

🎯 Exam Tip: Clearly define each term (work, energy, power) individually, and then explain the relationship (energy enables work, power is the rate of doing work) to provide a comprehensive answer.

Question 10: What do you understand by electrical energy? How is electricity obtained from thermal power plant?
Answer: Electrical energy is the energy that results from the flow of electric charge (electrons) through a conductor. It is a fundamental form of energy used to power most modern devices and systems. In a thermal power plant, electricity is generated through a series of energy conversions. First, fossil fuels (like coal, oil, or natural gas) are burned to produce heat energy. This heat boils water to create high-pressure steam. The steam is then channeled to spin large turbines, converting thermal energy into kinetic energy. The spinning turbines are connected to electrical generators, which convert the kinetic energy into electrical energy, which is then supplied to homes and industries.
In simple words: Electrical energy is power from moving electric charges. Thermal power plants make it by burning fuel to create steam, which spins machines to make electricity.

🎯 Exam Tip: When describing electricity generation in a thermal plant, outline the chain of energy transformations: Chemical (fuel) \( \rightarrow \) Heat \( \rightarrow \) Kinetic (steam/turbine) \( \rightarrow \) Electrical (generator).

Question 11: How is electricity produced in hydel power plant?
Answer: In a hydel (hydroelectric) power plant, electricity is produced by using the potential energy of stored water. Large dams are constructed across rivers to create reservoirs, holding vast amounts of water at a significant height. When electricity is needed, water is released from these reservoirs through large pipes called penstocks. As the water flows downwards, its potential energy is converted into kinetic energy. This fast-moving water then strikes the blades of a turbine, causing it to rotate. The turbine is connected to an electric generator, which converts the kinetic energy of the rotating turbine into electrical energy. This clean energy is then transmitted to consumers.
In simple words: Hydropower plants make electricity by letting water stored behind a dam flow down to spin turbines, which then power generators.

🎯 Exam Tip: The key principle of a hydroelectric plant is the conversion of gravitational potential energy of water into kinetic energy, then into mechanical energy of turbines, and finally into electrical energy.

Question 12: What can be done to reduce dissipation of electric energy?
Answer: To reduce the dissipation of electric energy, several measures can be taken:

• Switch off electrical appliances when they are not in use to prevent standby power consumption.
• Use energy-efficient lighting options like LED or CFL bulbs instead of traditional incandescent bulbs.
• Opt for electrical appliances with a higher star rating, as these are designed to consume less energy.
• Ensure proper maintenance of electrical appliances to keep them running efficiently.

This helps to make our use of electricity more efficient.
In simple words: To save electricity, turn off unused appliances, use LED/CFL bulbs, and buy appliances with high energy star ratings.

🎯 Exam Tip: Focus on practical, everyday actions and choices in appliance technology when discussing ways to reduce energy dissipation.

Question 13: What can be done to make air-conditioning more effective in homes?
Answer: To make air-conditioning more effective in homes and reduce energy waste, several steps can be taken. Using hollow bricks for walls provides better insulation, reducing heat transfer into the house. Ensuring that doors and windows are properly sealed helps prevent cool air from escaping and warm air from entering. Regular maintenance of the air conditioning unit, including cleaning filters and checking refrigerant levels, significantly improves its efficiency and cooling performance.
In simple words: To make air conditioning work better, use insulating bricks for walls, seal windows and doors, and regularly clean the AC unit.

🎯 Exam Tip: Focus on both structural elements (insulation, sealing) and maintenance (regular cleaning) to give a complete answer for improving AC effectiveness.

Question 14: What is electric power? How is consumption of electricity calculated for households? Explain with suitable example.
Answer: Electric power is the rate at which electrical energy is transferred or consumed. It describes how quickly electrical work is done. It can be expressed as \( P = \frac{\text{Work}}{\text{time}} \) or \( P = VI \) (Voltage \( \times \) Current).
For households, electricity consumption is typically measured in kilowatt-hours (kWh), which are also called "units". One unit of electricity means \( 1 \) kilowatt of power used for \( 1 \) hour.
To calculate consumption: Energy consumed \( (\text{in kWh}) = \text{Power of appliance (in kW)} \times \text{Time (in hours)} \).
Example: If a \( 1000 \,W \) (or \( 1 \,kW \)) bulb is used for \( 1 \) hour, it consumes \( 1 \,kW \times 1 \,h = 1 \,kWh \), which is \( 1 \) unit. If a \( 100 \,W \) bulb is used for \( 10 \) hours, it also consumes \( 0.1 \,kW \times 10 \,h = 1 \,kWh \), or \( 1 \) unit. This shows how household electricity bills are calculated based on these units. Also, \( 1 \,kWh \) is equivalent to \( 3.6 \times 10^6 \) Joules.
In simple words: Electric power is how fast electricity is used. Homes calculate electricity use in "units" (kilowatt-hours), which is power multiplied by time.

🎯 Exam Tip: Clearly define power, then explain the practical unit (kWh) and its calculation with a simple example. Emphasize the difference between Watt (power) and Watt-hour (energy).

Question 15: Explain conversion of energy when we switch on an electric bulb.
Answer: When an electric bulb is switched on, electrical energy is converted into two main forms: light energy and heat energy. The electricity flows through a filament (usually made of tungsten) inside the bulb. Due to the high electrical resistance of the filament, it heats up significantly, becoming incandescent (red-hot) and then emitting light. This process is known as the heating effect of electric current. While the primary purpose is to produce light, a substantial portion of the electrical energy is also converted into heat, which is a form of energy dissipation, making traditional bulbs less efficient than modern LED lights.
In simple words: When you turn on a light bulb, electricity changes mostly into light, but also into heat because of a hot wire inside.

🎯 Exam Tip: Explain the conversion process by mentioning the filament, its resistance, the heating effect, and the resulting light and heat energy outputs.

Work, Energy and Power Long Answer Type Questions

Question 2. A machine moves an object of 40 kg mass to 10 m height. Find the amount of work done.
Answer: For this machine, the mass of the object is \( 40 \text{ kg} \), the height it is moved to is \( 10 \text{ m} \), and the acceleration due to gravity is \( 9.8 \text{ m/s}^2 \). We can calculate the work done using the formula for potential energy, as the work done against gravity equals the change in potential energy.
Work done \( W = m \times g \times h \)
\( W = 40 \text{ kg} \times 9.8 \text{ m/s}^2 \times 10 \text{ m} \)
\( W = 3920 \text{ J} \)
\( W = 3.92 \text{ kJ} \)
In simple words: To find the work done when lifting an object, multiply its mass by gravity and the height it is lifted. This gives you the total energy used for lifting.

🎯 Exam Tip: Remember that work done against gravity is a form of potential energy, so use the formula \( W = mgh \) for such problems.

Question 3. An object of mass 6 kg falls from a height of 5 m. Find the change in its potential energy. (g = 10 m/s2)
Answer: When an object falls, its potential energy changes. The change in potential energy is equal to its initial potential energy when it was at the highest point, because its potential energy becomes zero at the ground.
Given mass \( m = 6 \text{ kg} \), height \( h = 5 \text{ m} \), and acceleration due to gravity \( g = 10 \text{ m/s}^2 \).
Potential energy \( PE = m \times g \times h \)
\( PE = 6 \text{ kg} \times 10 \text{ m/s}^2 \times 5 \text{ m} \)
\( PE = 300 \text{ J} \)
In simple words: The potential energy of an object at a certain height can be found by multiplying its mass, the gravity, and its height. When it falls, this potential energy changes into kinetic energy.

🎯 Exam Tip: The change in potential energy for a falling object is often calculated as the initial potential energy since the final potential energy at ground level is zero.

Question 5. When a spring is pulled up to 0.02 m then work done is 0.4 J. Find the spring constant.
Answer: We can find the spring constant using the formula for work done on a spring. The work done in compressing or stretching a spring is given by \( W = \frac{1}{2}kx^2 \), where \( k \) is the spring constant and \( x \) is the displacement. In this problem, the work done \( W = 0.4 \text{ J} \) and the displacement \( x = 0.02 \text{ m} \).
\( W = \frac{1}{2}kx^2 \)
\( 0.4 \text{ J} = \frac{1}{2} \times k \times (0.02 \text{ m})^2 \)
\( 0.4 = \frac{1}{2} \times k \times 0.0004 \)
\( 0.8 = k \times 0.0004 \)
\( k = \frac{0.8}{0.0004} \)
\( k = 2000 \text{ N/m} \)
In simple words: We can calculate how stiff a spring is by knowing the work done to stretch it and how much it stretched. The stiffer the spring, the more force it needs to stretch.

🎯 Exam Tip: Always remember that the displacement \( x \) in the spring work formula must be squared, and units should be consistent (Joules for work, meters for displacement, N/m for spring constant).

Question 6. An engine moves an object of 200 kg mass to a height of 50 m in 10 second. Find the power of the engine, (g = 10 m/s²)
Answer: First, we need to calculate the work done by the engine, which is the potential energy gained by the object. Then, we can find the power by dividing the work done by the time taken. Here, mass \( m = 200 \text{ kg} \), height \( h = 50 \text{ m} \), time \( t = 10 \text{ s} \), and \( g = 10 \text{ m/s}^2 \).
Work done \( W = m \times g \times h \)
\( W = 200 \text{ kg} \times 10 \text{ m/s}^2 \times 50 \text{ m} \)
\( W = 100000 \text{ J} \)
Now, calculate power:
Power \( P = \frac{\text{Work}}{\text{time}} \)
\( P = \frac{100000 \text{ J}}{10 \text{ s}} \)
\( P = 10000 \text{ W} \)
\( P = 10 \text{ kW} \)
In simple words: Power tells us how fast work is done. To find the engine's power, first calculate the total work done (energy to lift the object), then divide that by how much time it took.

🎯 Exam Tip: Power is the rate at which work is done or energy is transferred. Ensure all units are in SI before calculation: mass in kg, height in meters, time in seconds.

Question 7. 5 electrical appliances are used for 10 hours per day in a home. If two appliances are of 200 W and three ap N then find number of units consumed by these appliances in a day.
Answer: Let's assume the "three ap N" is a typo and refers to three appliances each consuming N Watts. The original document had "three ap Typesetting math: 83% N", which suggests N is the wattage for these three appliances. Since N is undefined, this problem cannot be fully solved numerically. However, if we assume 'N' means '200 W' like the first two appliances (a common pattern in such problems), or if 'N' is a placeholder for another value not provided, we must use the given numbers.
Units per day = \( \frac{\text{Total Power in Watts} \times \text{Hours}}{1000} \)
Two appliances: \( 2 \times 200 \text{ W} = 400 \text{ W} \)
Three appliances: Let's assume they are also 200W for a solvable problem: \( 3 \times 200 \text{ W} = 600 \text{ W} \)
Total power = \( 400 \text{ W} + 600 \text{ W} = 1000 \text{ W} \)
Total usage hours = \( 10 \text{ hours} \)
Units consumed per day = \( \frac{1000 \text{ W} \times 10 \text{ h}}{1000} = 10 \text{ units} \).
If 'N' stands for '100W' as in some similar examples: three appliances \( 3 \times 100 \text{ W} = 300 \text{ W} \). Total power \( 400 \text{ W} + 300 \text{ W} = 700 \text{ W} \). Units consumed per day = \( \frac{700 \text{ W} \times 10 \text{ h}}{1000} = 7 \text{ units} \).
Since 'N' is not specified, we cannot give a precise numerical answer without further information. The answer provided in the source is '16 units', which implies the appliances have different wattages than assumed. The source's output for this question: "Units per day = 1.6 x 10 h = 16". This shows that the original wattage for the appliances (200W for two and something else for three) must have resulted in a total power of 1.6 kW (1600 W).
In simple words: To calculate electricity units, you need the total power of all appliances in kilowatts multiplied by how many hours they run. One unit means 1 kilowatt-hour of electricity used.

🎯 Exam Tip: When given appliance wattages, convert total power to kilowatts (divide by 1000) before multiplying by hours to find energy consumed in kilowatt-hours (units).

Question 8. An object of 40 kg mass is moving with velocity 2 m/s. A force applied on it increases its velocity to 5 m/s. Find the work done by force.
Answer: The work done by the force is equal to the change in kinetic energy of the object. We can use the work-energy theorem to find this. The mass \( m = 40 \text{ kg} \), initial velocity \( u = 2 \text{ m/s} \), and final velocity \( v = 5 \text{ m/s} \).
Work done \( W = \frac{1}{2}m(v^2 - u^2) \)
\( W = \frac{1}{2} \times 40 \text{ kg} \times ((5 \text{ m/s})^2 - (2 \text{ m/s})^2) \)
\( W = 20 \times (25 - 4) \)
\( W = 20 \times 21 \)
\( W = 420 \text{ J} \)
In simple words: When a force makes something speed up, the work done by that force is the extra energy the object gets. You find this by calculating the change in its movement energy.

🎯 Exam Tip: Remember the work-energy theorem: the net work done on an object is equal to the change in its kinetic energy. This avoids needing to know the force and displacement separately.

Question 9. If an object of mass 50 kg is lifted to height of 3 m then find its potential energy. If this object falls freely from that height then find its kinetic energy when it is at halfway distance. (g = 10 m/s²)
Answer: First, calculate the potential energy when the object is at its maximum height. Then, consider the object falling to halfway. We know that mechanical energy is conserved, meaning the sum of kinetic and potential energy remains constant.
Given: Mass \( m = 50 \text{ kg} \), total height \( H = 3 \text{ m} \), \( g = 10 \text{ m/s}^2 \).
Potential energy at height \( H \):
\( PE_H = m \times g \times H \)
\( PE_H = 50 \text{ kg} \times 10 \text{ m/s}^2 \times 3 \text{ m} \)
\( PE_H = 1500 \text{ J} \)
At this maximum height, the kinetic energy is \( 0 \text{ J} \), so total mechanical energy \( ME = 1500 \text{ J} \).

At halfway distance, the height is \( h = \frac{H}{2} = \frac{3 \text{ m}}{2} = 1.5 \text{ m} \).
Potential energy at halfway:
\( PE_{half} = m \times g \times h \)
\( PE_{half} = 50 \text{ kg} \times 10 \text{ m/s}^2 \times 1.5 \text{ m} \)
\( PE_{half} = 750 \text{ J} \)

Since mechanical energy is conserved, \( ME = KE_{half} + PE_{half} \).
\( 1500 \text{ J} = KE_{half} + 750 \text{ J} \)
\( KE_{half} = 1500 \text{ J} - 750 \text{ J} \)
\( KE_{half} = 750 \text{ J} \)
In simple words: When an object is lifted, it stores potential energy. As it falls, this potential energy turns into kinetic energy. At any point during its fall, the total energy (potential + kinetic) stays the same.

🎯 Exam Tip: Conservation of mechanical energy is key for problems involving falling objects. The sum of potential and kinetic energy remains constant if only conservative forces (like gravity) are doing work.

Question 10. A block of 8 kg is moving on a frictionless surface with velocity 4 m/s. After compressing a spring, this block comes to rest. What is the compression in spring if spring constant is 2 x 104 N/m?
Answer: When the block comes to rest after compressing the spring, all its initial kinetic energy is converted into the potential energy stored in the spring. We can use the principle of conservation of energy here.
Given: Mass \( m = 8 \text{ kg} \), velocity \( v = 4 \text{ m/s} \), spring constant \( k = 2 \times 10^4 \text{ N/m} \).
Initial Kinetic Energy of the block:
\( KE = \frac{1}{2}mv^2 \)
\( KE = \frac{1}{2} \times 8 \text{ kg} \times (4 \text{ m/s})^2 \)
\( KE = 4 \times 16 \)
\( KE = 64 \text{ J} \)

Potential Energy stored in the spring:
\( PE_{spring} = \frac{1}{2}kx^2 \)
Where \( x \) is the compression in the spring.

By conservation of energy, \( KE = PE_{spring} \).
\( 64 \text{ J} = \frac{1}{2} \times (2 \times 10^4 \text{ N/m}) \times x^2 \)
\( 64 = 10^4 \times x^2 \)
\( x^2 = \frac{64}{10^4} \)
\( x^2 = \frac{64}{10000} \)
\( x = \sqrt{\frac{64}{10000}} \)
\( x = \frac{8}{100} \)
\( x = 0.08 \text{ m} \)
In simple words: The energy of the moving block is fully used to push the spring. The amount the spring is squashed depends on how fast the block was moving and how stiff the spring is.

🎯 Exam Tip: This problem demonstrates the conversion of kinetic energy into elastic potential energy. Equating these two energy forms is the key to solving for unknown variables like compression or spring constant.

Work, Energy and Power Additional Questions Solved

I. Multiple Choice Questions

Question 1. The unit of work is joule. The other physical quantity that has same unit is
(a) power
(b) velocity
(c) energy
(d) force
Answer: (c) energy
In simple words: Both work and energy are measured using the unit called joule, because energy is the ability to do work.

🎯 Exam Tip: Remember that work and energy are closely related concepts in physics, often measured in the same units (Joules in SI system).

Question 2. The spring will have maximum potential energy when
(a) it is pulled out
(b) it is compressed
(c) both (a) and (b)
(d) neither (a) nor (b)
Answer: (c) both (a) and (b)
In simple words: A spring stores energy when it is either stretched or squeezed from its normal resting size. The more it's stretched or compressed, the more energy it holds.

🎯 Exam Tip: Elastic potential energy stored in a spring depends on the square of its displacement from equilibrium, meaning both compression and extension lead to stored energy.

Question 3. The gravitational potential energy of an object is due to
(d) All of the options
Answer: (d) All of the options
In simple words: Gravitational potential energy depends on an object's mass, the acceleration due to gravity, and its height above a reference point. All these factors together determine the stored energy.

🎯 Exam Tip: Gravitational potential energy is calculated using \( PE = mgh \), where \( m \) is mass, \( g \) is acceleration due to gravity, and \( h \) is height. All these contribute to its value.

Question 4. A ball is dropped from a height of 10 m.
(a) Its potential energy increases and kinetic energy decreases during the falls.
(b) Its potential energy is equal to the kinetic energy during the fall.
(c) The potential energy decreases and the kinetic energy increases during the fall.
(d) The potential energy is 'O' and kinetic energy is maximum while it is falling.
Answer: (c) The potential energy decreases and the kinetic energy increases during the fall.
In simple words: As a ball falls, its height gets smaller, so its potential energy goes down. At the same time, it speeds up, so its kinetic energy goes up. The total energy stays the same.

🎯 Exam Tip: Remember the principle of conservation of mechanical energy: as an object falls, potential energy converts to kinetic energy, keeping the total constant (ignoring air resistance).

Question 5. If the velocity of a body is doubled its kinetic energy
(a) gets doubled
(b) becomes half
(c) does not change
(d) becomes 4 times
Answer: (d) becomes 4 times
In simple words: Kinetic energy depends on the square of speed. So, if you double the speed of something, its kinetic energy becomes four times bigger.

🎯 Exam Tip: The formula for kinetic energy is \( KE = \frac{1}{2}mv^2 \). This means if \( v \) doubles, \( v^2 \) becomes four times larger, so \( KE \) also becomes four times larger.

Question 6. How much time will be required to perform 520 J of work at the rate of 20 W?
(a) 24 s
(b) 16 s
(c) 20 s
(d) 26 s
Answer: (d) 26 s
In simple words: Power is how fast work is done. To find the time taken, divide the total work by the power.

🎯 Exam Tip: Use the formula \( \text{Time} = \frac{\text{Work}}{\text{Power}} \). Ensure units are consistent (Joules for work, Watts for power, seconds for time).

Question 7. A student carries a bag weighing 5 kg from the ground floor to his class on the first floor that is 2 m high.
(c) 100 J
Answer: (c) 100 J
In simple words: To lift the bag, the student does work against gravity. This work equals the bag's mass multiplied by gravity and the height it was lifted. Assuming \( g = 10 \text{ m/s}^2 \), the work is \( 5 \text{ kg} \times 10 \text{ m/s}^2 \times 2 \text{ m} = 100 \text{ J} \).

🎯 Exam Tip: When a question about lifting an object to a height is given, and the answer is in Joules, it is typically asking for the work done against gravity (which equals the change in potential energy).

Question 8. The work done is '0' if
(a) The body shows displacement in the opposite direction of the force applied.
(b) The body shows displacement in the same direction as that of the force applied.
(c) The body shows a displacement in perpendicular direction to the force applied.
(d) The body moves obliquely to the direction of the force applied.
Answer: (c) The body shows a displacement in perpendicular direction to the force applied.
In simple words: Work is only done when a force causes movement in the same direction as the force. If the movement is sideways to the force, no work is done by that force.

🎯 Exam Tip: Work done is calculated as \( W = F \times d \times \cos\theta \). If the angle \( \theta \) between force and displacement is \( 90^\circ \) (perpendicular), then \( \cos 90^\circ = 0 \), resulting in zero work.

Question 9. One unit of electrical energy is equal to
(a) 3.6x 105 J
(b) 3.6 x 106 J
(c) 36 x 105 J
(d) both (b) and (c)
Answer: (d) both (b) and (c)
In simple words: One unit of electricity, also called a kilowatt-hour, is equal to 3.6 million Joules. Both 3.6 x 10^6 J and 36 x 10^5 J represent the same large number.

🎯 Exam Tip: Remember the conversion: \( 1 \text{ kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ J} \). Also, \( 36 \times 10^5 \text{ J} \) is the same as \( 3.6 \times 10^6 \text{ J} \).

Question 10. Which method is used to produce electricity in hydro electric power plant?
(a) By boiling the water to produce steam
(b) By ionizing water
(c) By running dynamo by kinetic energy of water
(d) Any of the options
Answer: (c) By running dynamo by kinetic energy of water
In simple words: Hydroelectric power plants make electricity by using the force of moving water to spin big machines called dynamos or turbines.

🎯 Exam Tip: In hydroelectric power, the potential energy of water stored at height is converted to kinetic energy, which then rotates turbines to generate electricity.

Question 12. In which of the following kinetic energy is converted into electrical energy?
(a) Tidal energy
(b) Hydro electric
(c) Wind energy
(d) All of these
Answer: (d) All of these
In simple words: In all these types of power plants – tidal, hydro, and wind – the movement energy (kinetic energy) from water or wind is used to create electricity.

🎯 Exam Tip: Understand the energy transformations: in all these cases, a natural flow (tide, river, wind) creates motion (kinetic energy), which is then converted into electrical energy via turbines and generators.

Question 13. Which of the following is the ultimate source of energy for us?
(a) LPG
(b) Nuclear
(c) Solar
(d) CNG
Answer: (c) Solar
In simple words: The sun is the main source of almost all energy on Earth. Sunlight helps plants grow (food), creates wind, drives the water cycle, and forms fossil fuels over millions of years.

🎯 Exam Tip: Consider the origin of various energy forms; most can be traced back to solar energy, directly or indirectly.

Work, Energy and Power Very Short Answer Type Questions

Question 1. Define work.
Answer: When a force acts on an object and makes it move over a distance in the direction of the force, we say that work has been done on the object. The formula for work is \( W = F \times s \), where \( F \) is force and \( s \) is displacement.
In simple words: Work means moving something using force. If you push a box and it moves, you have done work.

🎯 Exam Tip: A key aspect of defining work is that displacement must occur in the direction of the applied force; otherwise, no work is done by that specific force.

Question 2. What is the unit of work done?
Answer: The standard international unit (SI unit) of work done is the Joule (J). One Joule is equal to one Newton-meter (N.m).
In simple words: The unit for work is called the Joule. It is named after a scientist who studied energy.

🎯 Exam Tip: Remember that Joule is the unit for both work and energy, signifying their interchangeability.

Question 3. Name two conditions required to do work.
Answer: Two main conditions must be met for work to be done:
1. A force must be applied to the object.
2. The object must move (be displaced) in the direction of the applied force.
In simple words: For work to happen, you need to push or pull something, and that something must actually move because of your push or pull.

🎯 Exam Tip: Emphasize both conditions: force and displacement in the same direction. Without either, no work is done, even if effort is exerted.

Question 5. What is potential energy?
Answer: Potential energy is the energy an object possesses because of its position or state. For example, an object held at a certain height above the ground has gravitational potential energy, and a stretched spring has elastic potential energy.
In simple words: Potential energy is stored energy. It's the energy an object has because of where it is or how it's set up, like a ball held high up.

🎯 Exam Tip: Clearly distinguish potential energy (stored energy due to position/state) from kinetic energy (energy of motion).

Question 6. What is the formula of work done and give the units of each symbol.
Answer: The formula for work done is:
\( W = F \times s \)
Where:
\( W \) is Work done, with units of Joules (J).
\( F \) is Force, with units of Newtons (N).
\( s \) is displacement, with units of Meters (m).
In simple words: The way to figure out work is to multiply the force by the distance something moves. The unit for work is Joules.

🎯 Exam Tip: Always specify the SI units for each variable in a formula to show a complete understanding.

Question 7. If the work done is 20 J and displacement is 2 m then find the force applied.
Answer: We can find the force applied using the work done formula: \( W = F \times s \). In this case, work done \( W = 20 \text{ J} \) and displacement \( s = 2 \text{ m} \).
\( W = F \times s \)
\( 20 \text{ J} = F \times 2 \text{ m} \)
\( F = \frac{20 \text{ J}}{2 \text{ m}} \)
\( F = 10 \text{ N} \)
In simple words: If you know how much work was done and how far something moved, you can figure out how much force was used by dividing the work by the distance.

🎯 Exam Tip: Rearrange the work formula \( W = Fs \) to solve for force \( F = W/s \) or displacement \( s = W/F \) as needed.

Question 8. State whether the given phenomenon is an example of work done: When we push a pebble lying on surface.
Answer: Yes, pushing a pebble lying on a surface is an example of work done. When you apply a force to the pebble and it moves through a distance, work is performed.
In simple words: When you push a pebble and it moves, you are doing work because you applied a force and it caused movement.

🎯 Exam Tip: For work to be done, there must be both an applied force and a displacement in the direction of the force. If the pebble doesn't move, no work is done.

Question 9. Give the formula of kinetic energy and potential energy.
Answer: The formulas for kinetic energy and potential energy are:
Kinetic Energy (\( KE \)): \( KE = \frac{1}{2}mv^2 \), where \( m \) is mass and \( v \) is velocity.
Potential Energy (\( PE \)): For gravitational potential energy, \( PE = mgh \), where \( m \) is mass, \( g \) is acceleration due to gravity, and \( h \) is height.
In simple words: Kinetic energy is how much energy something has because it's moving, while potential energy is the stored energy it has because of its position.

🎯 Exam Tip: Distinguish between the two types of energy by remembering that kinetic energy is always associated with motion, and potential energy with position or configuration.

Question 10. Name the energy stored when a rubber band is stretched?
Answer: When a rubber band is stretched, elastic potential energy is stored in it. This energy is stored due to the change in its shape from its normal state.
In simple words: When you stretch a rubber band, it stores energy called elastic potential energy. This energy can be released when the band returns to its original shape.

🎯 Exam Tip: Recognize elastic potential energy as a type of stored energy that comes from stretching, compressing, or twisting an elastic object.

Question 11. How many joules are there in 1 kj?
Answer: There are 1000 Joules in 1 kilojoule (kJ). The prefix 'kilo' always means a thousand.
\( 1 \text{ kJ} = 1000 \text{ J} \)
In simple words: Just like 1 kilogram is 1000 grams, 1 kilojoule is 1000 Joules.

🎯 Exam Tip: Remember common SI prefixes like kilo (1000), mega (1,000,000), milli (0.001) to easily convert between units.

Question 12. Define power.
Answer: Power is defined as the rate at which work is done or the rate at which energy is transferred. It tells us how quickly energy is being used or converted from one form to another.
\( P = \frac{W}{t} \)
In simple words: Power is a measure of how fast you do work or use energy. If you do a lot of work quickly, you have high power.

🎯 Exam Tip: Always link power to the concept of 'rate' – it's about speed of energy transfer, not just the amount of energy.

Question 13. What is the unit of power? Define it.
Answer: The standard international unit (SI unit) of power is the Watt (W).
One Watt is defined as the power of an agent which does work at the rate of 1 Joule per second \( (1 \text{ J/s}) \). This means if 1 Joule of energy is used every second, the power is 1 Watt.
In simple words: The unit for power is called a Watt. One Watt means one Joule of energy is used every second.

🎯 Exam Tip: Remember that Watts measure power (rate of energy use), while Joules measure energy (total amount of energy used).

Question 14. What is the commercial unit of energy? Define it.
Answer: The commercial unit of electrical energy, often used for household electricity bills, is the kilowatt-hour (kWh).
One kilowatt-hour (\( 1 \text{ kWh} \)) is defined as the amount of energy consumed when an electrical appliance with a power rating of 1 kilowatt (1000 Watts) is used for 1 hour.
In simple words: For buying and selling electricity, we use a unit called a kilowatt-hour. It's like using a 1000-watt machine for one hour.

🎯 Exam Tip: Understand that kWh is a unit of energy, not power. It combines power (kW) and time (h) to represent total energy used.

Question 15. Convert 1 kWh into joules.
Answer: To convert 1 kilowatt-hour (kWh) into Joules, we use the definitions of kilowatt and hour.
\( 1 \text{ kWh} = 1 \text{ kW} \times 1 \text{ h} \)
\( 1 \text{ kW} = 1000 \text{ W} = 1000 \text{ J/s} \)
\( 1 \text{ h} = 3600 \text{ s} \)
So,
\( 1 \text{ kWh} = (1000 \text{ J/s}) \times (3600 \text{ s}) \)
\( 1 \text{ kWh} = 3600000 \text{ J} \)
\( 1 \text{ kWh} = 3.6 \times 10^6 \text{ J} \)
In simple words: One unit of electricity, which is a kilowatt-hour, is equal to a very big number of Joules. It's 3.6 million Joules.

🎯 Exam Tip: This conversion is fundamental for understanding energy costs and relationships. Memorize \( 1 \text{ kWh} = 3.6 \times 10^6 \text{ J} \).

Question 17. Give one example where work done on an object is zero.
Answer: A common example is a person walking horizontally while carrying a heavy load on their head. In this situation, the force applied by the person on the load is upwards (to support it), but the displacement of the load is horizontal. Since the force and displacement are perpendicular to each other, the work done by the person on the load is zero.


In simple words: If you carry a bag straight across a room, you are pushing up on it, but the bag is moving sideways. Because the force and movement are at right angles, no work is done on the bag itself.

🎯 Exam Tip: Work requires a component of force in the direction of displacement. If force is perpendicular to displacement, the work done is zero.

Question 18. Give one example where work done on an object is negative.
Answer: Work done is negative when the force applied on an object is in the opposite direction to its displacement. For example, when a person walks down the stairs with a load on their head, the force applied on the load is upwards (to support it), but the displacement of the load is downwards. Since the force and displacement are in opposite directions, the work done by the person on the load is negative.


In simple words: Work is negative if you push something one way, but it moves the opposite way. For example, when you slow down a moving car, the brakes do negative work.

🎯 Exam Tip: Negative work signifies that the force is opposing the motion, causing the object to lose energy or slow down.

Question 19. In the above figure a pendulum is shown oscillating from A to B, B to C and C to A. In its one oscillation state the position. Where its kinetic energy is maximum.
Answer: When a pendulum is raised to a certain height, for example at point B, it has maximum potential energy and zero kinetic energy. As it is released and swings downwards, its potential energy is converted into kinetic energy. The pendulum attains its maximum kinetic energy at the lowest point of its swing, which is point A, where its speed is highest.
In simple words: A pendulum swings back and forth. It moves fastest at the very bottom of its swing (point A), which is where it has the most energy of movement (kinetic energy).

🎯 Exam Tip: For a simple pendulum, kinetic energy is maximum at the mean position (lowest point), and potential energy is maximum at the extreme positions (highest points of the swing).

Question 20. A man does 60 J of work in 6 seconds. Calculate the power.
Answer: Power is the rate at which work is done. We can calculate power by dividing the total work done by the time taken to do that work.
Given: Work done \( W = 60 \text{ J} \), Time taken \( t = 6 \text{ s} \).
Power \( P = \frac{\text{Work}}{\text{time}} \)
\( P = \frac{60 \text{ J}}{6 \text{ s}} \)
\( P = 10 \text{ W} \)
In simple words: To find how powerful someone or something is, you divide the amount of work they do by how long it takes them to do it.

🎯 Exam Tip: Ensure that work is in Joules and time is in seconds when calculating power in Watts; these are the standard SI units.

Question 21. What is a source of energy?
Answer: A source of energy is anything that is capable of providing a sufficient and useful amount of energy to do work or perform actions. Energy sources can be natural (like the sun, wind, or fossil fuels) or man-made.
In simple words: An energy source is anything that gives us power or fuel to make things work. It's where we get the energy from.

🎯 Exam Tip: When defining energy sources, mention both their capability to provide useful energy and examples of such sources.

Work, Energy and Power Short Answer Type Questions

Question 1. What is gravitational potential energy?
Answer: Gravitational potential energy is the energy stored in an object because of its position in a gravitational field, usually its height above a reference point. It is defined as the work done in raising an object to a specific point above the ground. Energy possessed by a body due to its velocity is called kinetic energy. This kinetic energy can be compared to potential energy as shown in the table below.

In simple words: Gravitational potential energy is the stored energy an object has because it's lifted higher. The higher it is, the more energy it stores, ready to be released when it falls.

🎯 Exam Tip: Focus on the 'position' aspect in your definition of potential energy, especially gravitational potential energy, and include its formula.

Question 3. Give two situations where energy is supplied but no work is done.
Answer: Here are two situations where energy is used but no work is done in the physics sense:
(i) A person pushing a heavy rock: If a person pushes a very heavy rock with all their strength, they use a lot of their own energy. However, if the rock does not move at all, then no work is done on the rock because there is no displacement.
(ii) A person standing with a heavy load: If a person stands still with a heavy load on their head, they are expending energy to keep the load balanced and supported. But since the load is not moving through a distance, no work is done on the load.
In simple words: You use energy, but no work is done if you push something hard and it doesn't move, or if you hold something heavy and stay still. Work needs movement.

🎯 Exam Tip: To illustrate zero work despite energy expenditure, choose examples where force is applied but displacement is absent, or where force is perpendicular to displacement.

Question 4. What do you understand by energy?
Answer: Energy is defined as the capacity or ability of an object or system to do work. It exists in various forms, such as kinetic energy, potential energy, heat energy, chemical energy, and electrical energy. When an object does work, it loses energy, and the object on which work is done gains energy. The standard unit for energy is the Joule.
In simple words: Energy is simply the ability to do things or make changes. If you have energy, you can push, pull, or move things.

🎯 Exam Tip: When defining energy, always mention its connection to 'doing work' and state the SI unit (Joule).

Question 5. Give two examples in your daily life where you can see kinetic energy is doing work.
Answer: Here are two examples from daily life where kinetic energy is performing work:
(i) Blowing wind: The kinetic energy of moving air (wind) can do work by rotating the blades of a windmill to generate electricity or by pushing a sailboat across water.
(ii) Rotating wheel: The kinetic energy of a rotating wheel, such as on a bicycle or a car, does work by enabling the vehicle to move forward, overcoming friction and air resistance.
In simple words: Moving air can turn a windmill, and spinning wheels help a car move. These are examples where movement energy is doing useful work.

🎯 Exam Tip: When giving examples, clearly identify the moving object and the 'work' or 'action' it performs due to its motion.

Question 7. What is energy? Name different forms of energy. Give the unit of energy.
Answer: Energy is the capacity of a body or system to do work. It is a fundamental concept in physics and exists in various forms. The unit of energy is the Joule (J).
Different forms of energy include:
- Potential energy (stored energy, e.g., gravitational, elastic)
- Kinetic energy (energy of motion)
- Heat energy (thermal energy)
- Chemical energy (energy stored in chemical bonds)
- Electrical energy (energy associated with electric charge)
- Nuclear energy (energy stored in atomic nuclei)
- Light energy (radiant energy)
- Sound energy (mechanical wave energy)
In simple words: Energy is the power to do things. It comes in many types like movement energy, stored energy, heat, and light. The main unit we use is the Joule.

🎯 Exam Tip: For this question, ensure your definition of energy is clear, you list at least 4-5 distinct forms, and you correctly state the SI unit.

Question 8. Two stones A and B of same mass fall from height \( h_1 \) and \( h_2 \) respectively on sand where \( h_2 > h_1 \). Which stone will exert more force on the sand and why? Name the energy present in it.
Answer: Stone B will exert more force on the sand. This is because Stone B falls from a greater height \( h_2 \) compared to Stone A which falls from height \( h_1 \). Since \( h_2 > h_1 \), Stone B will have more gravitational potential energy when it starts falling. As it falls, this greater potential energy is converted into greater kinetic energy, meaning Stone B will hit the sand with higher velocity and thus transfer more energy upon impact.
The energy present in the stones before they hit the sand is kinetic energy (due to their motion), which was transformed from their initial gravitational potential energy (due to their height).
In simple words: The stone dropped from higher up (Stone B) will hit the sand harder. This is because it started with more stored energy (potential energy) which turned into more movement energy (kinetic energy) by the time it hit the ground.

🎯 Exam Tip: Explain both the energy transformation (potential to kinetic) and the conservation principle; greater initial potential energy results in greater kinetic energy upon impact, leading to more force.

Question 9. Derive the formula for kinetic energy.
Answer: Let's consider an object with mass \( m \) moving with a uniform initial velocity \( u \). Now, a constant force \( F \) acts on it, causing a displacement \( s \) and changing its velocity to \( v \).
The work done \( W \) on the object is given by:
\( W = F \times s \) --- (i)

According to Newton's Second Law of Motion:
\( F = ma \)

From the third equation of motion, relating initial velocity \( u \), final velocity \( v \), acceleration \( a \), and displacement \( s \):
\( v^2 = u^2 + 2as \)
We can rearrange this to find \( s \):
\( 2as = v^2 - u^2 \)
\( s = \frac{v^2 - u^2}{2a} \)

Substitute \( F \) and \( s \) into equation (i):
\( W = (ma) \times \left(\frac{v^2 - u^2}{2a}\right) \)
\( W = m \times \frac{(v^2 - u^2)}{2} \)
\( W = \frac{1}{2}m(v^2 - u^2) \)

This formula represents the work done, which is equal to the change in kinetic energy. If the object starts from rest, then \( u = 0 \). In this case, the work done becomes:
\( W = \frac{1}{2}mv^2 \)
Therefore, the kinetic energy \( KE \) of an object of mass \( m \) moving with velocity \( v \) is given by:
\( KE = \frac{1}{2}mv^2 \)
In simple words: Kinetic energy is found by multiplying half its mass by its speed squared. This formula shows how a force moving an object changes its movement energy.

🎯 Exam Tip: When deriving formulas, clearly state the initial assumptions, laws (like Newton's second law), and kinematic equations used in each step.

Question 10. Derive the formula for potential energy.
Answer: To derive the formula for gravitational potential energy, consider an object with mass \( m \) that is lifted vertically upwards through a height \( h \) from the ground level.
The force required to lift the object is equal to its weight.
Force \( F = \text{weight} = mg \)

The work done (\( W \)) in lifting the object is given by the product of the force applied and the vertical displacement (height).
\( W = \text{Force} \times \text{displacement} \)
\( W = F \times h \)
Substituting \( F = mg \):
\( W = mgh \)

The energy gained by the object due to its position (height) is equal to the work done on it. This stored energy is its gravitational potential energy (\( PE \)).
Therefore, Potential Energy \( PE = W \)
\( PE = mgh \)


In simple words: When you lift something up, you do work against gravity. This work is stored in the object as potential energy, which is calculated by multiplying its mass, gravity, and height.

🎯 Exam Tip: The derivation of potential energy highlights that the work done to lift an object is stored as its potential energy, assuming no energy loss to friction.

Question 11. Explain, law of conservation of energy.
Answer: The law of conservation of energy states that energy can neither be created nor destroyed. It can only be transformed from one form to another, or transferred from one system to another, but the total amount of energy in an isolated system always remains constant. This means the total energy before any transformation or transfer is always equal to the total energy after it. For example, in a free-falling object, potential energy converts into kinetic energy, but their sum (mechanical energy) stays the same.
In simple words: This law means you can't make or destroy energy, you can only change it from one type to another, like how a battery stores chemical energy that turns into electrical energy.

🎯 Exam Tip: Clearly state that energy is conserved (neither created nor destroyed) and can only be transformed or transferred, providing a simple example to illustrate the concept.

Question 12. Two boys A and B were given a task to carry 20 kg load from ground level to height 10 m. A completed the work in 40 s and B in 60 s. Calculate the power in both the cases. Who has greater power?
Answer: First, we calculate the work done to lift the load. The mass of the load \( m \) is 20 kg, and the height \( h \) is 10 m. The acceleration due to gravity \( g \) is 10 m/s\( ^2 \).
Work done \( W = mgh \)
Work done \( W = 20 \text{ kg} \times 10 \text{ m/s}^2 \times 10 \text{ m} = 2000 \text{ J} \)

Now, we calculate the power for boy A and boy B.
For boy A: Time taken \( t_A = 40 \text{ s} \)
Power \( P_A = \frac{\text{Work done}}{\text{Time taken}} \)
Power \( P_A = \frac{2000 \text{ J}}{40 \text{ s}} = 50 \text{ W} \)

For boy B: Time taken \( t_B = 60 \text{ s} \)
Power \( P_B = \frac{\text{Work done}}{\text{Time taken}} \)
Power \( P_B = \frac{2000 \text{ J}}{60 \text{ s}} \approx 33.34 \text{ W} \)

Since \( 50 \text{ W} > 33.34 \text{ W} \), the power of boy A is greater than boy B.
In simple words: Work is how much energy is used. Power is how fast you use that energy. Both boys did the same amount of work, but Boy A did it faster, so Boy A had more power.

🎯 Exam Tip: Remember to clearly state the formulas for work and power, show all calculation steps, and correctly identify the units for each physical quantity (Joules for work, Watts for power).

Question 13. An electrical appliance of 100 W is used for 3 h per day. Calculate the units of energy consumed per day and for a month.
Answer: First, we convert the power from watts to kilowatts. \( 100 \text{ W} = 0.1 \text{ kW} \).
The appliance is used for 3 hours per day.

To find the energy consumed per day, we use the formula:
Energy consumed \( = \text{Power} \times \text{Time} \)
Energy consumed per day \( = 0.1 \text{ kW} \times 3 \text{ h} = 0.3 \text{ kWh} \)
Since 1 unit of electricity is equal to 1 kWh, the energy consumed per day is 0.3 units.

To find the energy consumed for a month (assuming 30 days), we multiply the daily consumption by 30:
Energy consumed per month \( = 0.3 \text{ units/day} \times 30 \text{ days} = 9 \text{ units} \).
In simple words: We find out how much power the appliance uses, then multiply it by how many hours it runs each day. This gives us the daily energy use. We then multiply that by the number of days in a month to find the total monthly energy used. Electricity bills are calculated this way.

🎯 Exam Tip: Always remember to convert watts to kilowatts and use hours for time when calculating energy consumption in units (kWh).

Question 14. A body possess potential energy of 460 J whose mass is 20 kg and is raised to a certain height. What is the height when \( g = 10 \text{ m/s}^2 \)?
Answer: We are given:
Potential Energy \( P.E. = 460 \text{ J} \)
Mass \( m = 20 \text{ kg} \)
Acceleration due to gravity \( g = 10 \text{ m/s}^2 \)
We need to find the height \( h \).

The formula for potential energy is:
\( P.E. = mgh \)
We can rearrange this formula to find \( h \):
\( h = \frac{P.E.}{mg} \)
Substitute the given values:
\( h = \frac{460 \text{ J}}{20 \text{ kg} \times 10 \text{ m/s}^2} \)
\( h = \frac{460}{200} \text{ m} \)
\( h = 2.3 \text{ m} \)
So, the height at which the object is raised from the ground is 2.3 m.
In simple words: Potential energy is stored energy because of an object's height. We know how much potential energy the object has, its mass, and gravity, so we can use a simple math formula to work out how high it must be.

🎯 Exam Tip: Ensure you correctly rearrange the potential energy formula to solve for height, and always include units in your calculations for clarity.

Question 15. Explain the working of a hydroelectric power plant to produce electricity.
Answer: To produce hydroelectricity, large dams are built on rivers to stop the water flow. This creates big reservoirs of water at a high level. The water from this high level in the dam is then sent through large pipes, which lead to turbines at the bottom of the dam. The force of the flowing water makes the turbines spin very fast. These turbines are connected to a generator. As the turbines rotate, the generator produces electricity. In this process, the potential energy of the stored water changes into kinetic energy as it flows, which then changes into mechanical energy in the turbine, and finally into electrical energy in the generator.
In simple words: Hydroelectric power plants use the energy of falling water to make electricity. Water stored in a dam falls through pipes, spinning a large fan called a turbine. This spinning turbine then turns a machine called a generator to create electricity. It's a clean way to produce power.

🎯 Exam Tip: When explaining power generation, emphasize the energy transformations at each stage: potential to kinetic, kinetic to mechanical, and mechanical to electrical energy.

Question 1. (a) Define power. What is the S.I. unit of power and commercial unit of power? (b) A tube light of 80 W is used for 8 hours per day. Calculate the unit of energy consumed in 1 day by the tube light.
Answer:
(a) Power is the rate at which work is done or the rate at which energy is transferred. It tells us how quickly energy is used.
The S.I. unit of power is the watt (W). One watt is defined as one joule of work done per second, so \( 1 \text{ W} = \frac{1 \text{ J}}{1 \text{ s}} \).
The commercial unit of power is the kilowatt-hour (kWh). This unit is commonly used for billing electricity consumption.

(b) The power of the electric tube light is 80 W.
To calculate energy in kilowatt-hours (kWh), we convert the power to kilowatts:
Power \( = 80 \text{ W} = \frac{80}{1000} \text{ kW} = 0.08 \text{ kW} \).
The time used per day is 8 hours.
Energy consumed per day \( = \text{Power} \times \text{Time taken} \)
Energy consumed per day \( = 0.08 \text{ kW} \times 8 \text{ h} = 0.64 \text{ kWh} \).
Since 1 unit of energy is equal to 1 kWh, the energy consumed by the tube light in 1 day is 0.64 units.
In simple words: Power tells us how fast energy is used. For a tube light, we change its power to kilowatts and multiply by the hours it runs to find how many electricity "units" it uses in a day.

🎯 Exam Tip: Differentiate between the S.I. unit of power (watt) and the commercial unit (kilowatt-hour) and remember the conversion from watts to kilowatts for energy calculations.

Question 2. (a) List any 3 situations in your daily life where you can say work has been done. (b) What work is said to be done to increase the velocity of a car from 15 km/h to 30 km/h, if the mass of the car is 1000 kg?
Answer:
(a) Here are two situations where work is done:
(i) If you push a pebble lying on a surface and it moves a certain distance, then work is said to be done.
(ii) If a girl pulls a trolley and the trolley moves through a distance, then work is said to be done.

(b) To calculate the work done in increasing the velocity of a car, we use the work-energy theorem, which states that the work done is equal to the change in kinetic energy.
Mass of the car \( m = 1000 \text{ kg} \)
Initial velocity \( u = 15 \text{ km/h} \)
Final velocity \( v = 30 \text{ km/h} \)

Work done \( W = \text{Change in Kinetic Energy} \)
\( W = \frac{1}{2} m(v^2 - u^2) \)
We will use the numerical values as given in the source for calculation:
\( W = \frac{1}{2} \times 1000 \times ((30)^2 - (15)^2) \)
\( W = \frac{1}{2} \times 1000 \times (900 - 225) \)
\( W = \frac{1}{2} \times 1000 \times 675 \)
\( W = 500 \times 675 \)
\( W = 337500 \text{ J} \)
Therefore, the work done to increase the velocity of the car is 337500 Joules.
In simple words: (a) Work is done when a force moves something over a distance. (b) When a car speeds up, work is done to change its kinetic energy (energy of motion). We calculate this using the car's mass and how much its speed changed.

🎯 Exam Tip: Remember that work done is equivalent to the change in kinetic energy. For practical examples, work requires both a force and displacement in the direction of the force.

Question 3. Explain the following: (a) An object increases its potential energy when raised through a height. (b) Energy is neither created nor destroyed then from where do we get energy? (c) When we push the wall, the wall does not move and no work is done.
Answer:
(a) When an object with a certain mass \( M \) is lifted to a certain height, work is done against gravity. This work done is stored in the object as potential energy. So, the higher an object is raised, the more potential energy it gains. This stored energy can be converted into other forms later.

(b) The law of conservation of energy states that energy cannot be created or destroyed, it can only be transformed from one form to another. We get energy from various sources where it exists in different forms, such as the sun, planets, wind, and water. For example, the sun's energy is used by plants to make food through photosynthesis. When we eat green vegetables, this stored food energy enters our bodies, allowing us to perform activities.

(c) Work is defined as the product of force and displacement in the direction of the force. If you push a wall, you are applying a force. However, if the wall does not move (meaning there is no displacement), then according to the definition of work, no work is done on the wall, even if you feel tired. This is a key concept in physics.
In simple words: (a) Lifting an object stores energy in it called potential energy. (b) Energy doesn't just appear; it changes from one form to another, like sunlight becoming plant food. (c) If you push something but it doesn't move, no physics "work" is done.

🎯 Exam Tip: Clearly define each concept. For energy conservation, provide a real-world example of energy transformation. For work, emphasize that both force and displacement are essential for work to be done.

Question 4. State and explain one example where (i) Kinetic energy is present in a body and is used; and (ii) Potential energy is present in a body and is used.
Answer:
(i) Kinetic energy is the energy an object has due to its motion. An example where kinetic energy is present and used is a moving car. The car's engine converts chemical energy from fuel into kinetic energy, making the car move. This kinetic energy is then used to cover distances. Another example is a rotating wheel, which has kinetic energy due to its spin.

(ii) Potential energy is the energy stored in an object because of its position or state. An example where potential energy is present and used is water stored behind a dam. The water at a certain height in the dam holds a large amount of potential energy. When this water is released and flows downwards, its potential energy is converted into kinetic energy, which can then be used to rotate turbines and generate electricity. This conversion demonstrates the practical use of stored potential energy.
In simple words: (i) Kinetic energy is about movement, like a moving car or a spinning wheel. (ii) Potential energy is stored energy, like water held high up in a dam, which can later be released to do work.

🎯 Exam Tip: When providing examples for energy types, clearly explain the source of the energy and how it is utilized or transformed to perform work.