Selina Concise Solutions for ICSE Class 8 Physics Chapter 4 Energy

1. When we are pushing a wall we are not doing any work as the position of wall is not change i.e. wall has not moved in the direction of force.
In physics, work is only considered to be done when there is actual movement caused by a force. Even if you feel tired from pushing a heavy stationary object, the scientific work done remains zero.
Teacher's Tip: No movement means no work, no matter how much effort you put in!
Exam Tip: To score full marks, always explain that displacement is zero in cases like pushing a wall.

2. WORK: “is said to be done if on applying force on a body, the body moves (or changes it position) from it place in the direction of force. W = F x d
Or
“Work is said to be done by a force applied on a body, if it changes its size or shape.”
Work is the transfer of energy through motion or deformation. It is calculated by multiplying the force used by the distance the object travels while that force is being applied.
Teacher's Tip: Think of work as "Force ×s Distance".
Exam Tip: Remember that work can also be done when you squash or stretch an object, changing its shape.

3. FACTORS AFFECTING THE AMOUNT OF WORK DONE : W = F x d
(i) Magnitude of force applied.
(ii) Distance moved by the body in the direction of force.
The amount of work increases if you either push with more strength or if the object travels a longer path. These two variables are directly proportional to the total work accomplished.
Teacher's Tip: Work is like a product of effort and travel.
Exam Tip: Mention both the 'force' and 'displacement' when listing the factors that determine work done.

4. UNIT OF WORK : W = F x d
s.i unit W= 1N x 1m = Nm = joule (J)
1 kgf = 9.8 N is force on 1 kg
therefore F = mg
Work done - 1 kgf x m = 9.8 N m = 9.8 J = 10 J nearly
The Joule is the standard scientific unit used to measure the energy transferred when a force of one Newton moves an object by one meter. We use Joules because it simplifies calculations across different branches of science.
Teacher's Tip: Remember that 1 Joule = 1 Newton ×s 1 Meter.
Exam Tip: Always use capital 'J' as the symbol for Joule in your numerical answers.

5. A cooli standing with a box on his head, does no work as distance moved is zero.
Standing still with a heavy load requires muscular effort, but since the box does not change its position, no work is performed on the box. This is a classic example used to distinguish between physical tiredness and scientific work.
Teacher's Tip: If you don't go anywhere, you didn't do "work" in your physics book!
Exam Tip: Identify the lack of 'displacement' as the reason for zero work in stationary load cases.

6. A cooli with a box on his head and walking is doing no work as force is acting vertically downward and direction of motion is at right angle.
When the force (lifting the box) and the motion (walking forward) are at 90 degrees to each other, no work is done against gravity. This is because the force is not causing the movement in the direction it is being applied.
Teacher's Tip: Pushing "Up" while moving "Forward" equals zero work done on the load.
Exam Tip: Mention that the angle between force and displacement is 90° to explain this phenomenon.

7. ENERGY: “is capacity of doing work.”
Or
“The work done on a body in changing its state is called energy.”
s.1. unit of energy = S.I. unit of work = (J)
Energy is like a "bank account" that allows an object to perform actions or move other things. Because work and energy are essentially the same thing in different forms, they share the exact same unit of measurement.
Teacher's Tip: Think of energy as the "fuel" needed to perform work.
Exam Tip: Defining energy as the 'capacity to do work' is the most accurate way to score marks.

8. JOULE: “A body is said to possess a energy of one joule. If a force of 1 Newton moves the body by a distance of 1 metre in the direction of force.”
This provides a specific definition of the unit Joule based on mechanical action. It sets a standard for how much energy is required for basic physical tasks.
Teacher's Tip: 1 J = 1 N ×s 1 m - it's the "One-One-One" rule.
Exam Tip: Be sure to mention 'in the direction of force' when defining a Joule.

9. MECHANICAL ENERGY: “The energy possessed by a body due to its state of rest or state of motion is called mechanical energy.
Mechanical energy is the sum of an object's movement energy and its stored energy based on its position. It is the most common form of energy we observe in everyday machines and physical activities.
Teacher's Tip: Mechanical energy is the "Big Umbrella" that covers both moving and sitting objects.
Exam Tip: Remember that Mechanical Energy = Potential Energy + Kinetic Energy.

10. Potential energy and kinetic energy are mechanical energies.
These two types of energy can change into one another, such as when a falling ball speeds up. They are the primary components that make up the total physical energy of any system.
Teacher's Tip: PE and KE are like two sides of the same mechanical coin.
Exam Tip: If a question asks for types of mechanical energy, list both Potential and Kinetic.

11. POTENTIAL Energy (P.E.) : “Is energy possessed by body due to its state of rest or position.” P.E. = mgh
Potential energy is "stored" energy that an object has because of where it is located, like a book on a high shelf. The higher the object or the heavier it is, the more potential energy it holds.
Teacher's Tip: Think of Potential energy as energy that has the "potential" to do work later.
Exam Tip: Use the formula P.E. = m ×s g ×s h for calculations, where h is height.

12. KINETIC Energy (K.E.) : “Is energy possessed by body due to its motion.”
K.E. = 1/2 M V²
Kinetic energy is the energy of a moving object, like a rolling bowling ball or a flying bird. It depends greatly on how fast the object is moving and how much mass it has.
Teacher's Tip: If it's moving, it's Kinetic!
Exam Tip: Note that speed (v) is squared in the formula, meaning speed has a huge impact on energy.

13. GRAVITATIONAL POTENTIAL ENERGY: “When a stone or water is raised (lifted) from ground to a height, work is done against the force of gravity. This work is stored in the stone or water in the form of GRAVITATIONAL POTENTIAL ENERGY.”
Lifting an object requires effort to fight against Earth's pull, and that effort stays trapped in the object as energy. When you let the object go, this stored gravitational energy is what makes it fall back down.
Teacher's Tip: Lifting things is like "winding up" their gravity energy.
Exam Tip: Mention that work done 'against gravity' is what creates this specific type of energy.

14. A stretched bow, due to change in position possesses potential energy. When stretched bow is relreased the arrow comes in motion and due to motion possesses the kinetic energy and hits the body on which it strikes.
A bow stores elastic potential energy when you pull the string back and change its shape. Releasing the string transfers that stored energy into the arrow, turning it into movement energy.
Teacher's Tip: Stretching a rubber band or a bow is like "saving" work to use later.
Exam Tip: Use the bow and arrow example to explain energy conversion from Potential to Kinetic.

15. When a body at a hight, it possess P.E. = mgh. When it falls, height decreases and speed increases
therefore its P.E. decreases and K.E. increases.
As an object falls, it loses altitude (height) but gains velocity (speed). This is a perfect example of energy being conserved as it simply swaps from a "stored" state to a "moving" state.
Teacher's Tip: As height goes down, speed goes up - it's an energy trade!
Exam Tip: In energy conversion questions, state that total mechanical energy remains constant if air resistance is ignored.

16. Powder : “Rate of doing work”. P = W/t
(Note: Verbatim textbook text uses "Powder," likely a typo for "Power"). Power measures how fast work is being done or how quickly energy is being used. A more powerful motor can lift a weight much faster than a weak one.
Teacher's Tip: Power is just "Work with a stopwatch".
Exam Tip: Use the formula Power = Work / Time to find how "fast" energy is spent.

17. S.I. unit of power = w/t
is J/Sec = JS^-1 = watt
The Watt is the standard unit for power, named after James Watt. One Watt is equal to doing one Joule of work every single second.
Teacher's Tip: 1 Watt = 1 Joule per second.
Exam Tip: Be careful with the spelling of 'Watt' and 'Work'; they start with the same letter but mean different things!

Test your self

A.Objective Questions

1. Write true or false for each statement

(a) A coolie does no work against the force of gravity while carrying a luggage on a road.
Answer: True.
While the coolie is moving forward, the force of gravity is pulling the luggage straight down. Since the motion is horizontal and the force is vertical, no work is done specifically against the vertical force of gravity.
Teacher's Tip: Work requires force and motion to be in the same line!
Exam Tip: Always specify that the angle is 90° between force and motion to justify zero work.

(b) The energy stored in water of a dam is the kinetic energy.
Answer: False.
The energy stored in water of a dam is the potential energy.
Water in a dam is held at a great height, which means it has gravitational potential energy waiting to be released. It only becomes kinetic energy once the gates open and the water starts rushing down.
Teacher's Tip: Static things at a height have "Potential," not Kinetic energy.
Exam Tip: If the water is 'stored' or 'still,' the answer is always Potential Energy.

(c) The energy of a flying kite is kinetic energy.
Answer: True.
Because the kite is actively moving through the air, it possesses energy due to that motion. (Note: It also has potential energy due to its height, but kinetic is correct for its moving state).
Teacher's Tip: Anything that "goes" has Kinetic energy.
Exam Tip: Mention that motion is the key reason why a flying kite has kinetic energy.

(d) Work done by a boy depends on the time in which he does work.
Answer: False.
Work depends only on the force used and the distance moved, regardless of how fast or slow it happens. Time is used to calculate power, not the total amount of work.
Teacher's Tip: Work doesn't care about the clock; Power does!
Exam Tip: Do not include 'time' in your formula or definition for Work.

(e) Power spent by a body depends on the time for which it does work.
Answer: True.
Power is the rate of work, so if you do the same job in less time, you have used more power. This is why faster machines are usually described as being more powerful.
Teacher's Tip: Power is "Speedy Work".
Exam Tip: State that power is inversely proportional to time for a given amount of work.

2. Fill in the blanks

(a) Work is said to be done by a forte only when the body moves.
Without movement (displacement), the scientific value of work is always zero. This highlights that work is an action that results in a physical change of position.
Teacher's Tip: Force + Displacement = Work.
Exam Tip: Use the word 'displacement' or 'moves' to complete this definition.

(b) Work done = Force x distance moved in direction of force.
This is the standard formula for work in its simplest form. It ensures that we only count the part of the movement that is actually pushed by our force.
Teacher's Tip: Direction matters! Push and move must be in the same line.
Exam Tip: Memorize W = F ×s d as your primary work formula.

(c) The energy of a body is its capacity to do work.
Energy is effectively "work in storage" or "work in progress." An object with zero energy cannot push or move anything else.
Teacher's Tip: No Energy = No Work.
Exam Tip: 'Capacity to do work' is the most common and correct definition for Energy.

(d) The S.I. unit of energy is joule.
Named after James Prescott Joule, this unit is used for all types of energy, including heat and electricity. It helps scientists compare different energy forms using a single standard.
Teacher's Tip: Joule is the "Global Currency" for energy.
Exam Tip: Joule is used for both Work and Energy; they are identical in units.

(e) The potential energy of a body is due to its state of rest or position and kinetic energy of body is due to its state of motion.
This sentence defines the two main branches of mechanical energy. Position gives potential, while velocity gives kinetic.
Teacher's Tip: PE = Place; KE = Kick (motion).
Exam Tip: Be careful to match 'rest/position' with Potential and 'motion' with Kinetic.

(f) Gravitational potential energy U = mass x force of gravity on unit mass x height.
This is a word version of the formula U = mgh. It shows how gravity and height work together to store energy in heavy objects.
Teacher's Tip: Think of it as "Mass ×s Gravity ×s Height".
Exam Tip: Ensure you include all three factors (m, g, and h) when explaining gravitational PE.

(g) Kinetic energy = 1/2 x mass x (speed)²
This formula calculates the exact amount of energy in a moving object. Because speed is squared, doubling your speed actually quadruples your kinetic energy!
Teacher's Tip: Speed has a much bigger effect on KE than mass does.
Exam Tip: Don't forget the "1/2" and the "squared" symbol in the KE formula.

(h) Power P= work done/time taken.
This formula allows us to see how efficient or strong a process is over a period. It is the core calculation for electrical and mechanical output.
Teacher's Tip: P = W / t.
Exam Tip: Use this formula when a question gives you "Work" and "Seconds".

(i) The S . i. unit of power is watt
The Watt measures work flow, similar to how a speedometer measures travel flow. Most lightbulbs and appliances are rated in Watts to show how much energy they use per second.
Teacher's Tip: 1 Watt = 1 Joule per second.
Exam Tip: Capitalize 'Watt' when writing the full word, but use 'W' for the symbol.

(j) I H.P. = 746 W
Horsepower is an older unit of power that is still used today for car engines and large motors. One H.P. is roughly equivalent to the power of a standard horse.
Teacher's Tip: 746 is the "Magic Number" to convert H.P. to Watts.
Exam Tip: Memorize this exact value (746) for unit conversion questions.

3. Match the following
 

Column A
(a) A stone at a height
(b) A moving ball
(c) Energy
(d) Power
(e) watt

Column B
(i) power
(ii) joule
(iii) work done in 1 s
(iv) potential energy
(v) kinetic energy

Answer:
(a) A stone at a height - (iv) potential energy
(b) A moving ball - (v) kinetic energy
(c) Energy - (ii) joule
(d) Power - (iii) work done in 1 s
(e) watt - (i) power
These pairings link physical states and concepts to their scientific energy forms and units. For instance, "work done in 1 s" is the literal definition of Power.
Teacher's Tip: Match objects to their "Energy Type" and units to their "Quantities".
Exam Tip: Draw clean, straight lines for matching to ensure your answers are readable.

4. Select the correct alternative

(a) The S.I. unit of work is
1. second
2. metre
3. joule
4. newton
Answer: 3. joule
Work and energy always use the Joule as their standard unit. Newton is for force, while second and metre are for time and distance.
Teacher's Tip: Joule = Work.
Exam Tip: Be careful not to pick 'Newton' (force) by mistake.

(b) No work is done by a force if the body
1. moves in direction of force
2. does not move
3. moves in opposite direction
4. none of the these
Answer: 2. does not move
As we learned, displacement is a mandatory part of work. If d = 0, then F ×s d = 0.
Teacher's Tip: "Static" objects have zero work done on them.
Exam Tip: The word 'No work' usually points to 'no displacement'.

(c) Two coolies A and B do some work in time 1 minute and 2 minute respectively. The power spent is
1. same by both coolies
2. is more by coolie A than by B
3. is less by coolie A than by B
4. nothing can be said.
Answer: 2. is more by coolie A than by B
Since coolie A finished the job faster (1 minute vs 2 minutes), he has a higher rate of work. Power is inversely related to time.
Teacher's Tip: Fast workers are "Powerful" workers.
Exam Tip: Remember that smaller time (t) means larger power (P) for the same work (W).

(d) The expression of power P is
1. P = mgh
2. P = P = 1/2 Mv²
3. P = F x d
4. P = F x d/t
Answer: 4. P = F x d/t
This combines the work formula (F ×s d) with the power formula (W / t). It shows how much force and distance are covered per unit of time.
Teacher's Tip: F ×s d / t is the same as "Work over Time".
Exam Tip: Learn both W/t and F ×s d/t as expressions for Power.

(e) I H.P. is equal to
1. 1 W
2. 1 J
3. 746 J
4. 746 W
Answer: 4. 746 W
This is a standard constant value used in engineering. It represents the power of one standard horse.
Teacher's Tip: 746 is the number you need for engine power conversions.
Exam Tip: Ensure you pick 'W' (Watts) and not 'J' (Joules) for this answer.

(f) When a boy doubles his speed, his kinetic energy becomes
1. half
2. double
3. four times
4. no change
Answer: 3. four times
Because velocity is squared in the formula (v²), doubling the speed (2v) results in 2² = 4 times the energy. This is why high-speed accidents are so dangerous.
Teacher's Tip: Double speed = Quadruple energy (2 ×s 2 = 4).
Exam Tip: Use the formula 1/2 mv² to prove the "four times" result in long answers.

(g) A boy lifts a luggage from height 2 m to 4 m. The potential energy will become
1. half
2. double
3. one-third
4. one-fourth
Answer: 2. double
Potential energy is directly proportional to height. Since 4 m is twice as high as 2 m, the energy is doubled.
Teacher's Tip: Double height = Double Potential Energy.
Exam Tip: Heights and PE always follow the same ratio if mass stays the same.

B. Short/Long Answer Questions

Question 1: Define work.
Answer: WORK “When a force is applied on a body and there is displacement of the body, work is said to be done.”
Work is the measurement of energy being used to move something. It combines the effort of the push with the success of the movement.
Teacher's Tip: Work is "Effort that results in Motion".
Exam Tip: Mention 'displacement' to get the full definition correct.

Question 2: When does a force perform work ?
Answer: Work is said to be done only when the force applied on a body i. makes the body more (/. e. there is a displacement of the body).
(Note: Verbatim OCR: "more" likely means "move"). A force performs work when it successfully causes an object to change its position in space. Without this change in position, the force is just a static push.
Teacher's Tip: Force must "win" and move the object for work to happen.
Exam Tip: Use the term 'displacement' as the necessary result of work.

Question 3: State two conditions when no work is done by a force.
Answer: Two conditions are :
(i) There should be no displacement i.e. S = 0
(ii) The displacement is NORMAL to the direction of FORCE i.e. - θ = 90°
Work is zero if either the object stays still or if the object moves sideways while you push down. These scientific rules explain why holding a heavy bag while walking is zero work against gravity.
Teacher's Tip: Zero move OR Zero angle = Zero Work.
Exam Tip: List both conditions separately for full credit.

Question 4: In which of the following cases is work being done : (a) A boy pushing a rock (b) A boy climbing up the stairs (c) A coolie standing with a box in his head (d) A girl moving on the road.
Answer: (b) A boy climbing up the stairs (d) A girl moving on the road.
Climbing involves moving your body's mass upwards against gravity, while moving on the road (walking) involves moving your mass over a distance. In both cases, motion occurs in the direction of applied force.
Teacher's Tip: Look for the cases where someone is actually "on the go".
Exam Tip: Eliminate options where the object is 'pushing a rock' (usually stationary) or 'standing' (no distance).

Question 5: A coolie is moving on a road with a luggage on his head. Does he perform work against the force of gravity ? Give reason for your answer.
Answer: A coolie carrying a luggage on his head moving on ground does i no work against the force of gravity as displacement is normal to the direction of force of gravity.
Gravity pulls straight down, but the coolie moves perfectly sideways. Because these two directions never meet, the weight of the bag doesn't "feel" the horizontal movement in terms of energy.
Teacher's Tip: Walking sideways doesn't "fight" gravity, it just ignores it!
Exam Tip: The keyword here is 'normal' or 'perpendicular' direction.

Question 6: The moon is revolving around the earth in a circular path. How much work is done by the moon ?
Answer: No work is done, since displacement is NORMAL to the direction | of force on the body. The force is CENTRIPETAL.
In a circle, the force pulls inward (centripetal) while the moon moves along the curve. At every point, the pull and the path are at 90 degrees, so no energy is spent on the orbit.
Teacher's Tip: Gravity holds the moon but doesn't "do work" to keep it moving.
Exam Tip: Mention 'centripetal force' and '90 degree displacement' for a perfect score.

Question 7: Write the expression for work done by a force,
Answer: Work done by applying force F is the product of force applied on the body and distance moved by the body in the direction of force , work done = Force x distance moved in the direction of force. W = F x d
This formula is the fundamental definition of mechanical work. It assumes that the force and the movement are happening along the same line.
Teacher's Tip: Work = Force times Distance.
Exam Tip: Always provide both the word formula and the symbol formula (W=Fd).

Question 8: State the S.I. unit of work and define it.
Answer: S.I. unit of work is Joule. Joule “Is that much work done when a force of 1 N displaces the body through a distance of 1 m in the direction of force.”
One Joule is the standard metric "packet" of work. It links the Newton of force to the meter of distance to create a consistent energy measurement.
Teacher's Tip: Remember the "1-1-1" rule for defining units.
Exam Tip: Specify 'in the direction of force' to complete the definition.

Question 9: State two factors on which the work done on a body depends.
Answer: Two factors are : (i) Magnitude of force applied (F). (ii) Distance moved by the body in the direction of force (d) or (5)
Work is a team effort between push and path. If you double the force or double the distance, you double the total work accomplished.
Teacher's Tip: Work depends on how hard and how far.
Exam Tip: Using the word 'magnitude' for force is more scientific.

Question 10: Define the term energy.
Answer: ENERGY : “Capacity of doing work” is called ENERGY
If a body has energy, it can apply force to move something else. If it has no energy, it is incapable of performing any physical work.
Teacher's Tip: Energy is like "Ability in the bank".
Exam Tip: This is the most popular 1-mark question in this chapter.

Question 11: State the S.I. unit of energy.
Answer: S.I unit __ is Joule (J).
Energy and work are essentially different stages of the same thing. Therefore, they share the Joule as their universal unit in the S.I. system.
Teacher's Tip: Work and Energy are "Twin Units".
Exam Tip: Don't forget that both use the same unit 'Joule'.

Question 12: Define 1 joule of energy.
Answer: Joule “is the capacity of a body to work of 1 J irrespective of time taken.”
This defines energy based on the amount of work it can produce. It highlights that the total amount of energy doesn't change based on how fast you use it.
Teacher's Tip: 1 Joule of energy can do 1 Joule of work!
Exam Tip: Mention that time does not affect the amount of energy defined.

Question 13: How is work related to energy ?
Answer: RELATION BETWEEN WORK AND ENERGY : “Energy is the capacity of doing work” Every form of energy → is work. i.e. work done on body is STORED IN THE FORM OF ENERGY. ENERGY is spent when a body does work. Thus to do more amount of work-more energy is needed.
Work is the process of using energy. When you do work on an object, you are transferring your energy into that object, which it can then store for its own use.
Teacher's Tip: Work is Energy in "Action".
Exam Tip: Use the term 'transfer of energy' to explain the relationship between these two.

Question 14: What are the two kinds of mechanical energy ?
Answer: Two KINDS OF MECHANICAL ENERGY : (i) The Potential energy (P.E.) (ii) The Kinetic energy (K.E.)
Mechanical energy is the total energy of an object's physical state. It combines the energy of being somewhere (Potential) and the energy of going somewhere (Kinetic).
Teacher's Tip: P.E. and K.E. are the two "Mechanical Brothers".
Exam Tip: List both PE and KE as the two components of mechanical energy.

Question 15: What is potential energy ? State its unit.
Answer: POTENTIAL ENERGY : (P.E. or U) “The energy possessed by a body due its position above the ground . or change in state.” UNIT : Unit of P.E. = S.I. UNIT OF ENERGY = Joule (J)
Potential energy is energy "waiting" to happen, stored because of an object's location or its stretched shape. Like all energy, it is measured in Joules.
Teacher's Tip: Potential = "Position or Shape Energy".
Exam Tip: Remember that both height and shape (like a spring) contribute to potential energy.

Question 16: Give one example of a body that has potential energy, in each of the following : (a) due to its position at a height, (b) due to its elongated stretched state.
Answer: (a) P.E. due to its position at a height : Water at a height has P.E. stored in it. Falling water from a height can be used to do work like turning a wheel.
(b) P.E. due to its elongated stretched state : A stretched rubber band (elongated state) has potential energy. It does work in restoring itself to its original state. A pebble placed on the stretched rubber catapujt, is thrown away when it is released to restore its original state.
Height potential energy depends on gravity, while stretched potential energy depends on the material's elasticity. Both can be released to create motion and do useful work.
Teacher's Tip: Think "Height" vs "Elastic".
Exam Tip: Use the 'stretched rubber band' example for part (b) as it is very clear.

Question 17: State two factors on which the potential energy of a body at a certain height above the ground depends.
Answer: Potential energy = mgh
therefore P.E. = m x h x g ‘g’ is constant depends upon m and h. Two factors on which P.E. depends : (i) Mass : greater the mass, greater is P.E. (ii) HEIGHT ABOVE THE GROUND : Higher the height of body, greater is the P.E.
Heavier objects and higher objects both contain more energy "in the bank." If you lift a brick higher, or lift a heavier stone to the same height, you increase the potential energy in both cases.
Teacher's Tip: P.E. likes heavy things and high places.
Exam Tip: State 'Mass' and 'Height' as the two primary variables.

Question 18: Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above the ground. Which of the two has greater potential energy ?
Answer: As g is constant and h is same in both the cases. Pe2 is greater than PE. Hence, Potential energy of body B (more mass) is greater than the P.E. of body A. Or As height of body A and is same and ‘g’ is constant, the body with greater mass i.e. body B has greater potential energy.
Since both objects are at the same altitude, the only difference is their mass. Because body B has twice as much matter, it stores twice as much energy at that height.
Teacher's Tip: More Mass = More Potential Energy.
Exam Tip: Justify your answer by mentioning that height is constant while mass varies.

Question 19: A bucket full of water is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy ?
Answer: As ‘g’ is constant in both cases and quantity of water (m) is same in both cases potential energy depends on height. Since height of second bucket kept at second floor is greater. Hence, second bucket at second floor has greater P.E.
In this scenario, the mass is identical, so the energy depends entirely on the altitude. The bucket on the second floor is further from the ground, giving it a higher "position" and more stored energy.
Teacher's Tip: High Floor = High Energy.
Exam Tip: Focus on 'height' being the deciding factor since 'mass' is identical.

Question 20: Write the expression for the gravitational potential energy explaining the meaning of the symbols used.
Answer: EXPRESSION FOR GRAVITATIONAL POTENTIAL ENERGY: P.E. = U = mgh. Where U is gravitational potential energy m is the mass of body. g is___ force of gravity on mass of 1 kg. mg___ is the force acting on body. h___ is the distance or height moved above the ground level.
This mathematical sentence shows that energy is the product of weight (mg) and distance (h). It perfectly mirrors the basic work formula (W = F ×s d).
Teacher's Tip: U = mgh is the "Recipe for Stored Energy".
Exam Tip: Label each symbol (m, g, h) clearly in your answer to get full marks.

Question 21: A body of mass m is moved from ground to a height h. If force of gravity on mass of 1 kg is g newton, find :
(a) the force needed to lift the body, (b) the work done in lifting the body and (c) the potential energy stored in the body.
Answer:
(a) When a body of mass m at A on ground is raised above ground through height h at B force is applied. This force applied = weight of body. Force on mass m = F = m g. Where ‘g’ is the force of gravity on a mass of 1 kg.
(b) Work done in lifting the body = Force x displacement h. W = mg x h.
(c) This work done W is stored in body in the form of Potential energy. P.E. mgh.
To lift something, you must push "up" at least as hard as gravity pulls "down." The energy you spend on that lift isn't lost; it turns into the potential energy the object now has at its new height.
Teacher's Tip: Force = Weight; Work = mgh; Energy = mgh.
Exam Tip: Show how (b) leads directly to (c) to demonstrate energy conservation.

Question 22: Define the term kinetic energy. Give one example of a body which possesses kinetic energy.
Answer: KINETIC ENERGY: “The energy possessed by a body by virtue of its motion is called KINETIC ENERGY.” Example : A bullet moving at high speed through has small mass, possesses kinetic energy and can penetrate the body. Or When a stretched bow is released the potential energy of arrow changes into kinetic energy and makes the arrow to move.
Kinetic energy is the power of movement. Whether it is a giant planet or a tiny bullet, anything with speed has the ability to smash into things and do work.
Teacher's Tip: If it's "zooming," it's Kinetic!
Exam Tip: Use the phrase 'by virtue of its motion' to define KE accurately.

Question 23: State two factors on which the kinetic energy of a moving body depends.
Answer: The energy possessed by a body by virtue of its motion is defined as kinetic energy. The factors on which it depends are (i) mass of the body. (ii) velocity of the body.
Heavier things and faster things both pack a bigger punch. Speed is especially important because it is squared in the scientific calculation of this energy.
Teacher's Tip: Mass and Speed are the "Batteries" of Kinetic Energy.
Exam Tip: Mention 'velocity' and 'mass' as the two primary factors.

Question 24: Two toy-cars A and B of masses 200 g and 500 g respectively are moving with the same speed. Which of the two has greater kinetic energy ?
Answer: CAR A mas = 200 g = 200/1000 = 1/5 kg = 0.2 kg. CAR B mass = 500 g = 500/1000 = 0.5 kg. K.E. = 1/2 mv².
because speed of both cars is same. K.E. of A = 1/2 x 0.2 x v² = 0.1 v². K.E. of B = 1/2 x 0.5 x v² = 0.25 v2. 0.25 v² is greater than 0.1 v²
therefore K.E. of car B is greater. Or Since speed of both cars is same.
therefore The speed of car having greater mass (i.e. of car B), the K.E. is greater.
therefore Kinetic energy of car B having greater mass is greater.
Even though they are traveling at the same speed, the heavier car has more "momentum" and energy. It takes more work to stop a heavier object moving at that speed, which proves it has more energy.
Teacher's Tip: Heavy + Same Speed = More Energy.
Exam Tip: Convert grams to kilograms before doing energy math to keep your units S.I.

Question 25: A cyclist double his speed. How will his kinetic energy change: increase, decrease or remain same ?
Answer: As Kinetic energy K.E. = 1/2 Mv². Since speed is doubled, its square will become 4 times.
therefore K.E. increases i.e. becomes 4 times. Or K.E j = Mv². When speed is doubled New speed v₁ = (2 v). New K.E.₂ = 1/2 m v₁²
therefore New K.E.₂ = 1/2 m (2v)². K.E.₂ = 1/2 m 4v² = 4 [1/2 m v²]. New K.E.₂ = 4 times K.E.₁.
Speed has a massive effect on energy because the formula squares the velocity number. Doubling the speed doesn't just double the energy; it quadruples it, making the cyclist much more powerful.
Teacher's Tip: Double the Speed = Four times the Energy (2 ×s 2 = 4).
Exam Tip: Use the 'v²' part of the formula to explain why energy grows so fast with speed.

Question 26: Write the expression for the kinetic energy of a body explaining the meaning of the symbols used.
Answer: Kinetic energy = 1/2 Mv². Where m is the mass of body v is the speed of body.
This formula allows us to put a number on motion energy. It shows that both mass and speed are needed to calculate the total energy of a moving object.
Teacher's Tip: K.E. = 0.5 ×s m ×s v ×s v.
Exam Tip: Always include the 1/2 at the start of the formula or you will get the wrong answer.

Question 27: A ball of mass m is moving with a speed v. What is its kinetic energy ?
Answer: Kinetic energy of a ball of mass m and moving with speed v is K.E. = 1/2 Mv².
The energy depends on these two factors (m and v). If the ball stops (v=0), its kinetic energy immediately drops to zero as well.
Teacher's Tip: This is the basic symbolic answer for a general KE question.
Exam Tip: Use the standard S.I. units (kg and m/s) when thinking about these symbols.

Question 28: Name the form of energy stored in a wound up spring of a watch.
Answer: It possesses Potential energy.
Because the spring has been twisted and its shape has been changed, it stores elastic energy. As the spring slowly returns to its normal shape, it releases this energy to move the clock hands.
Teacher's Tip: Springs store "Potential" energy by being squashed or twisted.
Exam Tip: Specify 'Elastic' Potential Energy for extra accuracy.

Question 29: Can a body possess energy even when it is not in motion ? Explain your answer with an example.
Answer: Yes, a body not in motion can possess energy. Example: Water stored in dam through not in motion possess potential energy. Or A stone at rest on the top of a building possesses P.E.
Energy is not just about moving; it can also be stored for later use. Position and shape are the two main ways stationary objects hold onto energy.
Teacher's Tip: Think of potential energy as energy "in waiting".
Exam Tip: Use 'Water in a dam' or 'Stone on a roof' as your primary examples.

Question 30: Name the type of energy (kinetic or potential) possessed by the following:
(a) A moving cricket ball. (b) A stone at rest on the top of a building. (c) A compressed spring. (d) A moving bus. (e) A bullet fired from a gun. (f) Water flowing in a river. (g) A stretched rubber band.
Answer:
(a) A MOVING BALL due to motion possesses KINETIC ENERGY.
(b) A stone at rest on the top of a building possesses POTENTIAL ENERGY,
therefore Due to height above ground.
(c) A compressed spring possesses potential energy due to changed position of spring.
(d) A moving bus possesses kinetic energy due to motion.
(e) A bullet fired from a gun possesses kinetic energy due to motion.
(f) WATER FLOWING IN A RIVER possesses kinetic energy due to its motion.
(g) A STRETCHED RUBBER BAND possesses potential energy due to changed position.
Items (a), (d), (e), and (f) are all Kinetic because they are moving. Items (b), (c), and (g) are Potential because they are either at a height or have been reshaped.
Teacher's Tip: Motion = Kinetic; Height/Stretching = Potential.
Exam Tip: Label the specific reason (like 'due to motion') next to your answer for better marks.

Question 31: Give an example to show the conversion of potential energy to kinetic energy when put in use.
Answer: A stretched bow has the potential energy because of its stretched position. When the stretched bow is released the potential energy of the bow changes into its kinetic energy.
The energy swaps from the string's tension to the arrow's movement. This transformation allows us to use stored mechanical energy to accomplish a goal like moving a projectile.
Teacher's Tip: Releasing a pull always changes PE into KE.
Exam Tip: A 'falling stone' is another simple example of this energy conversion.

Question 32: State the energy changes that occur in a watch spring while it unwinds.
Answer: A wound up watch spring has P.E. stored ¡n ¡t due to it wound up state. A the spring UNWINDS itself, the potential energy changes into KINETIC ENERGY with which it moves the arms of the watch.
The stored energy in the metal coil is slowly released to do the work of turning gears. This conversion keeps the watch running at a steady pace over time.
Teacher's Tip: Wound-up = Potential; Unwinding = Kinetic.
Exam Tip: Describe the transition as 'Potential Energy → Kinetic Energy'.

Question 33: Give reasons for the following:
(a) No work is done ¡fa man ¡s pushing against a wall.
(b) Hammer drives a nail into the wood only when it ¡s lifted up and then struck.
(c) A horse and a dog are running with the same speed. Which one of them has more kinetic energy than the other.
(d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work.
Answer: (a) As wall does not move from its place.-distance moved is zero. Hence, no work is done.
(b) On lifting the Hammer, its potential energy is stored in the hammer on striking the nail with hammer this energy is used in driving the nail into the wood. .
(c) A horse has more mass than dog. As both are running with the same speed. M1 of horse is greater than M2 of dog.
therefore K.E. of horse is more than K.E. of dog.
(d) A child reading a book while standing is not moving from its place i.e. displacement is zero. Hence product of force and displacement ¡s zero W=F x S W = mg x O = O. Hence, child is not doing any work. Where as teacher is moving from its place is doing work.
These examples show that work requires both effort and movement. Even though a student reading a book is working hard mentally, in physics, they aren't performing mechanical work because they are still.
Teacher's Tip: Work in science is only about "Physical results".
Exam Tip: For part (c), explain that 'more mass means more kinetic energy at the same speed'.

Question 34: State the energy changes in the following while ¡n use. (a) An electric bulb (b) An electric oven (c) A loud speaker (d) A microphone (e) An electric motor
Answer: (a) An electric bulb - Electrical to light energy
(b) An electric oven - Electrical to heat energy
(c) A loud speaker - Electrical to sound energy
(d) A microphone - Electrical to sound energy (Note: Actually Sound to Electrical)
(e) An electric motor - Mechanical to’aHcaI energy (Note: Actually Electrical to Mechanical).
Energy is constantly changing from one form to another to power our world. These devices act as converters that turn raw electricity into useful things like heat, light, or sound.
Teacher's Tip: Devices convert "Input" energy into "Output" energy.
Exam Tip: Use arrows (→) to show the conversion path clearly.

C. Numericals

Question 1: A force of 30 N acts on a body and moves it through a distance of 5 m ip the direction of force. Calculate the work done by the force.
Answer: F = 30 N d= 5m Work done = w = F x d W = 30 x 5= 150 J
We find the work by multiplying the strength of the push by the path traveled. In this case, 30 Newtons pushing for 5 meters creates 150 Joules of energy transfer.
Teacher's Tip: W = F ×s d is your best friend in math!
Exam Tip: Always show your substitution (30 ×s 5) and the final unit (J).

Question 2: A man lifts a mass of 20 kg to a height of 2.5 m. Assuming that the force of gravity on 1 kg mass is 10 N, find the work done by the man.
Answer: Mass = 20 kg h = 2.5 m Force of gravity on a mass of 1 kg = 10 N Force of gravity on a mass of 20kg F = mg = 20 x 10 = 200 N Work done in lifting the mass to height h = 20 m is (Note: Height is 2.5m) W = F x h = 200 N x 2.5 m = 200 x 25/10 = 500 J
To lift the load, the man must exert a force equal to its weight (200 Newtons). Carrying that force over a height of 2.5 meters results in a total work of 500 Joules.
Teacher's Tip: "Lifting" work is just weight times height.
Exam Tip: Calculate the force (mg) first before applying the work formula.

Question 3: A body when acted upon by a force of 10 kgf moves to a distance 0.5 m in the direction of force. Find the work done by the force. Take 1 kgf = 10 N.
Answer: F= 10 kgf = 10 x 10N= 100 N, Displacement S = 0.5 m Work done (i) When displacement is in the direction of force W = F x S W= 100 x 0.5 = 50 J
First, we convert the gravitational force unit (kgf) into the standard Newton unit (100 N). Then, multiplying by the half-meter distance gives us the work done in Joules.
Teacher's Tip: Always convert to Newtons before you calculate Joules.
Exam Tip: Show the conversion 10 ×s 10 = 100 N clearly in your steps.

Question 4: Two bodies of same masses are placed at heights h and 2h. Compare their gravitational potential energy.
Answer: Gravitational pot. energy of A / Gravitational pot. energy of B mgh / mg2h = 1/2 = 1 : 2
Because mass and gravity are the same for both, the energy depends only on the height. Since the second body is twice as high, it has exactly twice as much potential energy.
Teacher's Tip: Same mass + Double height = Double Potential energy.
Exam Tip: Write the final answer as a ratio (1:2) as requested by the word 'Compare'.

Question 5: Find the gravitational potential energy of 2.5 kg mass kept at a height of 15 m above the ground. The force of gravity on mass 1 kg is 10 n.
Answer: Mass m = 2.5 kg Gravitational potential energy is the work done against force of gravity ¡s stored in the body at a height h. P.E = U = mgh U = 2.5 x 10 x 15 U = 25/10 x 10 x 15 = 375 j
Using the standard mgh formula, we calculate the energy stored at that specific altitude. A 2.5 kg mass at 15 meters stores 375 Joules of potential energy.
Teacher's Tip: Gravity is our "multiplier" for potential energy.
Exam Tip: Show the breakdown of m, g, and h before doing the multiplication.

Question 6: The gravitational potential energy stored in a box of weight 150 kgf is 1.5 x 10⁴ J. Find the height of the box. Take l kgf = 10 N.
Answer: Gravitational potential energy U = mg x h 1.5 x 104 J = (150 kgf) x h 1.5 x 10⁴ J=(150 x IO N) x h h = 1.5 * 10⁴ / 1500 = 15 / 10 x 10000 / 1500 = 10 m
We rearrange the energy formula to solve for height by dividing the total energy by the object's weight. The box must be 10 meters high to store 15,000 Joules of energy.
Teacher's Tip: Height = Energy / (Mass ×s Gravity).
Exam Tip: Convert 1.5 ×s 10⁴ to 15,000 to make the division easier to see.

Question 7: The potential energy of a body of mass 0.5 kg increases by 100 J when it is taken to the top of a tower from ground. If force of gravity on 1 kg is 10 N, what is the height of the tower ?
Answer: Potential energy = (m g) h 100 J = (0.5 x 10) N x h h = 100 / (0.5 * 10) = 100 / 5 = 20 m
By knowing the energy "gain" and the weight of the object, we can find out how far up it was lifted. In this case, the 100 Joule increase corresponds to a 20-meter tall tower.
Teacher's Tip: Weight is the "Price" per meter of height gain.
Exam Tip: Divide Joules by (mass ×s gravity) to find height.

Question 8: A body of mass 60 kg is moving with a speed 50 m s_1. Find its kinetic energy.
Answer: m = 60 kg Speed v = 50 m S1 K.E = 1/2 MV² 1/2 x 60 x 50 x 50 = 75000 K.E = 75/10 x 1000 x 10 = 7.5 x 10⁴ J
Using the 1/2 mv² formula, we see how much movement energy a person-sized mass has at high speed. The result is a massive 75,000 Joules.
Teacher's Tip: Speed squared (50 ×s 50) is the biggest part of this math!
Exam Tip: Scientific notation (7.5 ×s 10⁴) is a great way to write large Joule answers.

Question 9: A truck of mass 1000 kg, increases its speed from 36 km h^-1 to 72 km h^-1 . Find the increase is its kinetic energy.
Answer: Weight of truck = Force = 1000 kgf.
therefore Mass of truck = 1000 kg. Initial speed u = 36 km h^-1 = 36 x 5/18 = 10 ms^-1.
Final speed v = 72 km h^-1 = 72 x 5/18 = 20 ms^-1.
Work done = Increase in energy = 1/2 mv² - 1/2 mu² = 1/2 m [(v + u) (v - u)] = 1/2 x 1000 (20 + 10) (20 - 10) = 1/2 x 1000 x 30 x 10.
Work done W = 150000 J = 1.5 x 105 J.
To find the energy "gain," we subtract the starting energy from the ending energy. Doubling the speed from 10 to 20 m/s adds a huge 150,000 Joules of kinetic energy to the truck.
Teacher's Tip: Convert km/h to m/s first by multiplying by 5/18.
Exam Tip: Use the formula 1/2 m(v² - u²) to find energy change in one step.

Question 10: A car is moving with a speed of 15 km h^-1 and another identical car is moving with a speed of 30 km h^-1. Compare their kinetic energy.
Answer: Two identical cars means, they have equal mass K.E = 1/2 mv² Let m be the mass of each car. Kinetic energy of car A = 1/2 x m x (15)²= 225/2 m.
Kinetic energy of car B = 1/2 M (30)² = 450 M J. K.E. of A / K.E. of B = 225 m / (2 * 450 m) = 1/4 = 1 : 4. K.E. of car B is 4 times K.E. of car A.
Since speed B is twice car A, and energy depends on the square of speed, car B has 2 ×s 2 = 4 times the energy. This comparison shows how much more dangerous higher speeds are.
Teacher's Tip: Velocity is squared, so ratios follow the square (2² = 4).
Exam Tip: You don't need the exact mass to find the ratio; 'm' will cancel out.

Question 11: A pump raises water by spending 4 x 10⁵ J of energy in 10 s. Find the power of pump.
Answer: Energy spent = w = 4 x 10⁵ J Time taken = 10 s Power = Energy / Time P = 4 x 10⁵ / 10 = 4 x 10⁴ W
Power is work per second. Dividing 400,000 Joules by 10 seconds gives us a power rating of 40,000 Watts.
Teacher's Tip: Power is how "fast" energy flows.
Exam Tip: 10⁵ divided by 10 simply becomes 10⁴ by subtracting one from the exponent.

Question 12: It takes 20 s for a girl A to climb up the stairs while girl B takes 15 s for the same job. Compare : (i) the work done and (ii) the power spent by then.
Answer: (i) As height is same for both girls A and B, work done is same (irrespective of time)
therefore Work done by A : Work done by B = 1 : 1.
(ii) Power = Energy / Time taken. Power of A / Power of B = Energy / t1 / Energy / t₂ = t₂ / t₁ = 15 / 20 = 3 : 4.
The "Work" is the same because the physical task is identical. However, girl B is more "Powerful" because she finished the task in less time, resulting in a 3:4 power ratio.
Teacher's Tip: Work is the "Task"; Power is the "Speed" of the task.
Exam Tip: For part (ii), remember that power and time have an 'inverse' relationship.