RBSE Solutions Class 8 Science Chapter 9 Work and Energy

Work And Energy Textbook Questions Solved

I. Multiple Choice Questions

 

Question 1. The unit of energy is:
(a) Newton
(b) Kilogram
(c) Joule
(d) Watt
Answer: (c) Joule
In simple words: Joule is the standard unit used to measure energy, telling us how much work can be done.

🎯 Exam Tip: Remember that "Watt" is a unit of power, not energy, and "Newton" is a unit of force. Distinguish carefully between these common physics units.

 

Question 2. The capacity of doing work is called:
(a) power
(b) force
(c) momentum
(d) energy
Answer: (d) energy
In simple words: Energy is simply the ability something has to do work or cause a change.

🎯 Exam Tip: Energy and work are closely related; work is done when energy is transferred or transformed.

 

Question 3. One of the following is not fossil fuel:
(a) petrol
(b) wood
(c) natural gas
(d) diesel
Answer: (b) wood
In simple words: Wood comes from trees and is a renewable resource, unlike fossil fuels which formed over millions of years from dead plants and animals.

🎯 Exam Tip: Fossil fuels like coal, petrol, and natural gas are non-renewable resources formed from ancient organic matter under heat and pressure.

 

Question 4. Which of the following apparatus used to convert electric energy into sound energy:
(a) electric motors
(b) electric magnet
(c) electric heater
(d) electric bell
Answer: (d) electric bell
In simple words: An electric bell uses electricity to make a hammer strike a gong, creating sound.

🎯 Exam Tip: Think about everyday devices and the energy transformations they perform. An electric heater converts electrical to heat energy, for example.

 

Question 3. Electric cell used in household, converts chemical energy into electric energy.
Answer: An electric cell, commonly known as a battery, works by changing stored chemical energy into electrical energy. This power then drives household devices. Batteries are essential for portable electronics because they can store energy efficiently.
In simple words: A battery uses chemicals to make electricity for home use.

🎯 Exam Tip: Remember that electric cells (batteries) are primary energy sources that convert chemical potential energy into electrical potential energy.

 

Question 4. Unit of energy is Joule.
Answer: The standard international (SI) unit for measuring energy is the Joule. One Joule is the amount of energy needed to apply a force of one Newton over a distance of one meter. Understanding energy units helps compare different forms of energy accurately.
In simple words: Energy is measured using a unit called the Joule.

🎯 Exam Tip: Knowing the correct SI units for physical quantities like energy, power, and force is crucial for exams.

 

Question 3. Match the column A with column B.

Answer:

In simple words: This table matches different ways energy changes from one form to another with the machines that make those changes happen. For example, a speaker changes electricity into sound.

🎯 Exam Tip: When matching columns, first identify the most obvious pairs, then use elimination for the remaining ones. Always double-check your connections.

 

Question 1. Define work.
Answer: Work is defined as the action where a force causes an object to move or displace in the direction of that force. When we push a box and it moves, we are doing work. It is a way of transferring energy.
In simple words: Work is done when you use force to move something.

🎯 Exam Tip: Remember that for work to be done, there must be both a force applied and a displacement in the direction of the force. No movement means no work.

 

Question 2. What are the things on which work done on the object depend?
Answer: The amount of work done on an object depends on two main things:
(i) The distance the object moves (displacement).
(ii) The amount of force applied on the object. Work is calculated by multiplying the force by the displacement. Work depends on how far an object moves and how much push or pull is used.
In simple words: Work depends on how much force you use and how far the object moves.

🎯 Exam Tip: The formula for work is Work = Force x Displacement. This shows the direct relationship between these three quantities.

 

Question 3. Give the two names which store kinetic energy and potential energy in it on the basis of daily life observation.
Answer: In daily life, we can observe objects storing both kinetic and potential energy. For example, water stored in overhead tanks has potential energy due to its height, and a flying bird has kinetic energy because it is moving. Also, a striking hammer and a running fan possess both types of energy. A striking hammer has kinetic energy from its motion and potential energy from being lifted. Similarly, a running fan has kinetic energy from its rotation and potential energy related to its position.
In simple words: Objects like water in a high tank (potential) and a moving bird (kinetic) show both types of energy. A hammer about to strike or a fan spinning also have both.

🎯 Exam Tip: Potential energy is stored energy due to position or state, while kinetic energy is the energy of motion. Many real-world examples combine both.

 

Question 4. How do we define heat energy conversation? Explain energy transfer on the basis of three different examples.
Answer: Heat energy conversion, or more commonly energy transformation, happens when energy changes from one form to another. For example, the energy stored in burning objects or hot objects is converted into other forms. Here are three examples of energy transfer:
(a) An electric heater changes electrical energy into heat energy, warming a room.
(b) A solar cell changes light energy from the sun into electrical energy, which can power devices.
(c) An electric generator changes mechanical energy (like from wind or water) into electrical energy, producing power for homes. All these conversions follow the law of conservation of energy.
In simple words: Heat energy conversion is when energy changes forms. Examples are a heater changing electricity to heat, a solar cell changing light to electricity, and a generator changing movement to electricity.

🎯 Exam Tip: Remember the Law of Conservation of Energy, which states that energy cannot be created or destroyed, only transformed from one form to another.

Work And Energy Long Answer Type Questions

 

Question 1. What do you mean by global energy crisis? What are the measures adopted to resolve the global energy crisis? Describe it in detail.
Answer: The global energy crisis refers to the problem of not having enough energy resources to meet the world's increasing demand, especially as many traditional sources like fossil fuels are limited and cause pollution. This shortage leads to higher prices and environmental damage. To solve this crisis, several important steps can be taken:
1. We should use electric appliances only when they are truly needed.
2. Always switch off electric appliances when nobody is in the room to save energy.
3. Using solar cookers and pressure cookers can help save cooking energy.
4. It's important to avoid wasting traditional energy sources, as they are non-renewable.
5. Turn off vehicle engines while waiting at traffic lights or for someone.
6. Make the best use of solar energy, which is a clean and renewable source.
7. Utilize biotic energy resources, such as biogas, which come from living matter.
8. Design buildings using techniques that keep them cool in summer and warm in winter, reducing the need for air conditioning and heaters.
9. Replace old incandescent bulbs with LED bulbs and tube lights because LEDs use much less electricity.
10. Regularly check and service vehicles to ensure they run efficiently and save fuel. Adopting these practices helps ensure energy availability for future generations.
In simple words: The global energy crisis means we are running out of energy and need to use it wisely. We can save energy by using less electricity, using solar power, turning off engines, and maintaining vehicles.

🎯 Exam Tip: For questions on global issues, always define the problem first, then list practical and sustainable solutions, categorizing them if possible (e.g., individual actions, technological solutions).

 

Question 2. Explain the differences between conventional energy sources and non-conventional energy sources with providing suitable example.
Answer: Energy sources are categorized into conventional and non-conventional based on their availability and environmental impact. Conventional sources have been used for a long time but are limited, while non-conventional sources are newer and more sustainable. The main differences are:

For example, coal is a conventional source, while solar power is a non-conventional source. It's important to use both responsibly.
In simple words: Conventional energy sources like coal are old, limited, and cause pollution. Non-conventional sources like solar power are new, unlimited, and clean.

🎯 Exam Tip: When comparing energy sources, focus on key differences like renewability, environmental impact, and long-term sustainability. Providing examples strengthens your answer.

Work And Energy Additional Questions Solved

I. Multiple Choice Questions

 

Question 1. When the displacement produced in a body is perpendicular to the direction of applied force then work done is:
(a) 1 Joule
(b) Zero Joule
(c) 103 Joule
(d) None of the options
Answer: (b) Zero Joule
In simple words: If you push something sideways but it only moves up or down, no work is done in the direction of your push. Work is zero when force and movement are at a right angle.

🎯 Exam Tip: Work done is calculated using W = Fd cos θ. When the angle (θ) between force and displacement is 90 degrees, cos 90° = 0, so work done is zero.

 

Question 2. Energy possessed by virtue of its position is called-
(a) Mechanical energy
(b) Chemical energy
(c) Kinetic energy
(d) Potential energy
Answer: (d) Potential energy
In simple words: Potential energy is the stored energy an object has because of its place or position.

🎯 Exam Tip: Think of a ball held high; it has potential energy that turns into kinetic energy when dropped. This relationship is key to understanding energy forms.

 

Question 3. Which of the following is not a fossil fuel?
(a) Fire wood
(b) Natural gas
(c) Wind energy
(d) Kerosene
Answer: (c) Wind energy
In simple words: Wind energy comes from the movement of air, which is a renewable source, unlike fossil fuels that formed over millions of years.

🎯 Exam Tip: Fossil fuels include coal, petroleum (like kerosene), and natural gas, which are formed from ancient organic matter under heat and pressure. Wind is a renewable resource.

 

Question 4. Which of the following converts chemical energy into electrical energy?
(a) Solar cell
(b) Bulb
(c) Battery
(d) Dynamo
Answer: (c) Battery
In simple words: A battery uses chemical reactions inside it to make electricity.

🎯 Exam Tip: Understand the energy transformations in common devices: a solar cell converts light to electrical, a bulb converts electrical to light/heat, and a dynamo converts mechanical to electrical.

 

Question 6. What type of energy is used to run submarine?
(a) Wind energy
(b) Ocean energy
(c) Atomic energy
(d) None of the options
Answer: (c) Atomic energy
In simple words: Many submarines use atomic (nuclear) energy because it can power them for very long times without needing to refuel.

🎯 Exam Tip: Atomic energy provides a high power output for a long duration, making it suitable for applications requiring extended operation without external energy sources, like submarines or spacecraft.

 

Question 7. Energy obtained from the Sun, wind, water, bio waste, oceans, underground energy is known as
(a) Non- conventional energy
(b) Conventional energy
(c) Ocean energy
(d) None of the options
Answer: (a) Non- conventional energy
In simple words: Energy from natural, constantly renewing sources like the sun, wind, and water is called non-conventional energy.

🎯 Exam Tip: Non-conventional energy sources are also known as renewable energy sources and are important for sustainable development as they do not get depleted.

 

Question 8. The energy of tides and currents in ocean can be converted into electrical energy, such energy is known as:
(a) Mechanical energy
(b) Ocean energy
(c) Atomic energy
(d) None of the options
Answer: (b) Ocean energy
In simple words: The power we get from the moving water in oceans, like tides and currents, is called ocean energy.

🎯 Exam Tip: Ocean energy includes tidal energy, wave energy, and ocean thermal energy conversion (OTEC), all harnessing the power of the sea.

 

Question 9. The unit of energy is
(a) Watt
(b) Kilometre/hour
(c) Joule/sec
(d) Watt/sec
Answer: (a) Watt
In simple words: While Joule is the standard unit for energy, Watt is often used in contexts like power consumption where energy transfer over time is considered. Watt measures how quickly energy is used or produced.

🎯 Exam Tip: It is important to remember the difference between energy and power. Energy is the capacity to do work (measured in Joules), while power is the rate at which energy is used or produced (measured in Watts).

 

Question 10. The combination of kinetic energy and potential energy is
(a) Electrical energy
(b) Mechanical energy
(c) Electric energy
(d) Thermal energy
Answer: (b) Mechanical energy
In simple words: Mechanical energy is the total energy an object has due to its movement (kinetic) and its position (potential).

🎯 Exam Tip: Mechanical energy is a fundamental concept in physics, representing the sum of an object's energy of motion and its stored energy due to position or configuration.

Work And Energy Very Short Answer Type Questions

 

Question 1. Define the term energy. Give its S.I unit.
Answer: Energy is defined as the capacity or ability to do work. It is what allows objects to move, change temperature, or undergo various transformations. The S.I. unit for energy is the Joule. Energy is crucial for all physical processes around us.
In simple words: Energy is the ability to do work, and its unit is the Joule.

🎯 Exam Tip: Clearly defining key terms and stating their correct SI units is essential for scoring full marks in physics questions.

 

Question 2. Name the place, where the atomic power plant is located in Rajasthan.
Answer: The atomic power plant in Rajasthan is located at Rawatbhata, which is in the Chittorgarh district. This plant plays a significant role in providing electricity to the region. It is one of India's major nuclear power stations.
In simple words: The atomic power plant in Rajasthan is at Rawatbhata, in Chittorgarh.

🎯 Exam Tip: Knowing the locations of major power plants is important for geography and general knowledge, especially concerning energy resources.

 

Question 3. What is the full form of LPG?
Answer: The full form of LPG is Liquefied Petroleum Gas. It is a flammable mixture of hydrocarbon gases used as fuel in heating appliances, vehicles, and as a refrigerant. LPG is a popular cooking fuel in many households. It is typically stored in cylinders under pressure.
In simple words: LPG stands for Liquefied Petroleum Gas, which is a common cooking gas.

🎯 Exam Tip: Many common abbreviations are important to know in science and general studies, such as LPG, CNG, and PVC.

 

Question 5. Define wind mill. Give its two uses.
Answer: A windmill is a machine that uses the power of wind to convert wind energy into mechanical energy. This mechanical energy can then be used for various purposes. Two common uses of windmills are:
1. Grinding wheat into flour.
2. Lifting water from wells for irrigation or other uses. Windmills are an ancient technology that has been modernized for generating electricity.
In simple words: A windmill is a machine that uses wind to create power, mainly for grinding wheat and pumping water.

🎯 Exam Tip: Windmills harness a renewable energy source (wind) and are an eco-friendly alternative to fossil fuels for mechanical work and electricity generation.

 

Question 6. What are renewable energy sources?
Answer: Renewable energy sources are natural resources that replenish themselves over a relatively short period, meaning they can be used continuously without running out. These are also known as non-conventional energy sources. Important examples include solar energy, wind energy, biomass, and biogas. Using renewable sources helps reduce pollution.
In simple words: Renewable energy sources are natural resources like sun, wind, and plants that can be used again and again because they replenish themselves.

🎯 Exam Tip: Focus on the key characteristic of renewable sources: their ability to naturally replenish, making them sustainable for long-term use.

 

Question 7. What is the other name of conventional energy sources?
Answer: Conventional energy sources are also known as non-renewable energy sources. These are resources that cannot be easily replaced once used, as they take millions of years to form. Examples include coal, petrol, water (in some contexts like hydroelectric dams where flow can be limited), and nuclear power (which relies on finite uranium). These sources are widely used but face depletion.
In simple words: Conventional energy sources are also called non-renewable energy sources because they cannot be easily replaced once used up.

🎯 Exam Tip: Differentiate clearly between "conventional/non-renewable" (like fossil fuels) and "non-conventional/renewable" (like solar, wind) energy sources.

 

Question 8. Define fossil fuel.
Answer: Fossil fuels are natural fuels formed over millions of years from the buried remains of plants and animals. They were created under intense pressure and high temperatures deep within the earth's crust. Common examples include coal, petroleum, and natural gas. These fuels are a primary source of energy globally but are finite and release greenhouse gases when burned.
In simple words: Fossil fuels are natural fuels like coal and oil that formed from ancient plants and animals over millions of years.

🎯 Exam Tip: Remember the three main fossil fuels: coal, petroleum, and natural gas, and that they are formed from organic matter over geological time.

 

Question 9. Write at least five form of energy.
Answer: Energy exists in many different forms, and it can often be converted from one form to another. Here are five common forms of energy:
1. **Mechanical Energy:** The energy of motion (kinetic) or position (potential) of an object.
2. **Heat Energy:** The energy associated with the random motion of atoms and molecules.
3. **Light Energy:** A form of electromagnetic radiation that our eyes can detect.
4. **Electrical Energy:** The energy associated with the flow of electric charge.
5. **Chemical Energy:** Energy stored in the bonds of chemical compounds, released during chemical reactions.
Other forms include nuclear energy and sound energy. All these forms are interconnected.
In simple words: Five types of energy are mechanical, heat, light, electrical, and chemical energy.

🎯 Exam Tip: When listing forms of energy, try to provide a brief distinguishing characteristic for each. Examples of each form can also be helpful.

Work And Energy Short Answer Type Questions

 

Question 1. What is bio-mass energy? What are its uses?
Answer: Biomass energy comes from organic matter like animal waste, cow dung, wild plants, and dry leaves. This material is fermented in a special tank called a digester, which produces biogas (also known as Gobar gas). Biogas burns with a clean, smokeless flame, making it a non-polluting fuel. Its main uses are as cooking fuel in homes and for lighting. This renewable energy source helps manage organic waste effectively.
In simple words: Biomass energy is made from animal waste and plants to create biogas, which is used for cooking and lighting without causing pollution.

🎯 Exam Tip: Highlight that biomass is a renewable energy source and its conversion into biogas offers both energy and waste management benefits, making it an environmentally friendly option.

 

Question 2. Define the following terms:
1. Ocean energy
2. Atomic energy
Answer:
1. **Ocean Energy:** This is the energy generated by the natural movements of the ocean, such as tides (both low and high), ocean waves, and currents. This mechanical energy is then converted into electrical energy. Ocean energy offers a vast, untapped renewable resource.
2. **Atomic Energy:** Also known as nuclear energy, this energy is released either by splitting (fission) or combining (fusion) the nuclei of atoms. Electricity is generated from atomic energy in nuclear reactors. It is a powerful but complex energy source. Both types of energy have significant potential.
In simple words: Ocean energy comes from sea movements like tides and waves. Atomic energy comes from splitting or joining tiny parts of atoms, used to make electricity.

🎯 Exam Tip: For definitions, provide a clear, concise explanation and, if appropriate, mention how the energy is generated or utilized.

 

Question 3. 1. Define work. Give its S.I unit. Write an expression for positive work done. 2. Calculate the work done in pushing a cart through a distance of 50 m against the force of friction equal to 250 N. Also state the type of work done.
Answer:
1. Work is said to be done when a force applied on an object causes a displacement in the object in the same direction as the force. This means energy is transferred to the object. The S.I. unit of work done is Joule. An expression for positive work done is Work = Force \( \times \) Displacement.
2. To calculate the work done:
Given Force (F) = 250 N
Displacement (s) = 50 m
Work done = F \( \times \) s
Work done = 250 N \( \times \) 50 m
Work done = 12500 Joule
In this case, the work done is negative because the work is done *against* the force of friction. When work is done against an opposing force, energy is lost from the system performing the work.
In simple words: 1. Work is using force to move something, measured in Joules. Positive work happens when force and movement are in the same direction. 2. If you push a cart for 50 meters against a friction of 250 Newtons, you do 12500 Joules of negative work because you are working against the friction.

🎯 Exam Tip: Remember that work done against a force like friction is considered negative because it removes energy from the system or the object. Always pay attention to the direction of force relative to displacement.