Master work, potential energy, kinetic energy, power and energy transformations through clear explanations, searchable answers and step-by-step practice.
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Work
Force producing displacement
W = F × d⬆
Potential Energy
Stored due to position
P.E. = mgh➜
Kinetic Energy
Energy due to motion
K.E. = ½mv²⚡
Power
Rate of doing work
P = W ÷ t
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Chapter coverage
What you will revise
Work and its S.I. unit
Potential and kinetic energy
Energy transformations
Power and numericals
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52 questions shown
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B Section
Short Answers
Concept definitions and direct responses
25 questions
01
Define work.
Short Answers
Work is said to be done when a force applied on a body produces displacement in the direction of the force.
02
When does a force perform work?
Short Answers
A force performs work when it causes a body to move in the direction of the force.
03
State two conditions when no work is done by a force.
Short Answers
When no force acts on the body.
When the body does not move despite the force acting on it.
04
In which of the following cases is work being done?
Short Answers
(a) A boy pushing a heavy rock: No work (if the rock does not move).
(b) A boy climbing up the stairs: Work is done.
(c) A coolie standing with a box on his head: No work.
(d) A girl moving on the road: Work is done.
05
Write the expression for work done by a force.
Short Answers
W = F × d
where, W = work done, F = force applied, d = displacement in the direction of force.
06
State the S.I. unit of work and define it.
Short Answers
S.I. unit of work is joule (J).
One joule is the work done when a force of 1 N moves a body through 1 m in the direction of the force.
07
State two factors on which the work done on a body depends.
Short Answers
Magnitude of force applied.
Distance moved in the direction of the force.
08
Define the term energy.
Short Answers
Energy is the capacity (ability) to do work.
09
State the S.I. unit of energy.
Short Answers
joule (J).
10
Define 1 joule of energy.
Short Answers
One joule of energy is the energy possessed by a body capable of doing one joule of work.
11
How is work related to energy?
Short Answers
Energy is measured by the amount of work done. Work done on a body equals the energy gained by it.
12
What are the two kinds of mechanical energy?
Short Answers
Potential Energy
Kinetic Energy
13
What is potential energy? State its unit.
Short Answers
Potential energy is the energy possessed by a body due to its position or state. Unit: joule (J)
14
Give one example of a body that has potential energy.
Short Answers
(a) Due to position at a height: Water stored in a tank.
(b) Due to elongated stretched state: A stretched rubber band.
15
Which bucket has greater potential energy?
Short Answers
The bucket kept on the second floor has greater potential energy because it is at a greater height.
16
Write the expression for gravitational potential energy.
Short Answers
P.E. = mgh
where, m = mass of body, g = acceleration due to gravity, h = height above the ground.
17
Define kinetic energy. Give one example.
Short Answers
Kinetic energy is the energy possessed by a body due to its motion. Example: A moving car.
18
State two factors on which kinetic energy depends.
Short Answers
Mass of the body.
Speed (velocity) of the body.
19
Two toy cars A and B of masses 200 g and 500 g move with the same speed. Which has greater kinetic energy?
Short Answers
Car B (500 g) has greater kinetic energy because kinetic energy is directly proportional to mass.
20
A cyclist doubles his speed. How will his kinetic energy change?
Short Answers
Kinetic energy becomes four times because K.E. ∝ v².
21
Write the expression for kinetic energy.
Short Answers
K.E. = ½ mv²
where, m = mass of body, v = speed of body.
22
A ball of mass m is moving with speed v. What is its kinetic energy?
Short Answers
K.E. = ½ mv²
23
Name the form of energy stored in a wound-up spring of a watch.
Short Answers
Elastic potential energy.
24
Name the type of energy possessed by:
Short Answers
(a) Moving cricket ball – Kinetic energy
(b) Stone at rest on top of a building – Potential energy
(c) Compressed spring – Potential energy
(d) Moving bus – Kinetic energy
(e) Bullet fired from a gun – Kinetic energy
(f) Water flowing in a river – Kinetic energy
(g) Stretched rubber band – Potential energy
25
State the energy changes:
Short Answers
(a) Electric bulb: Electrical → Light + Heat
(b) Electric oven: Electrical → Heat
(c) Loudspeaker: Electrical → Sound
(d) Microphone: Sound → Electrical
(e) Electric motor: Electrical → Mechanical
(f) Windmill: Wind (kinetic) → Electrical
C Section
Long Answers
Detailed explanations and reasoning
6 questions
26
State two factors on which potential energy depends.
Long Answers
Mass of the body.
Height of the body above the ground.
27
Two bodies A and B have masses 10 kg and 20 kg at the same height. Which has greater potential energy?
Long Answers
Body B has greater potential energy because P.E. = mgh and its mass is greater.
28
A body of mass m is moved from ground to height h. Calculate force needed, work done, and energy stored.
Long Answers
(a) Force needed = mg
(b) Work done = Force × Distance = mg × h = mgh
(c) Potential energy stored = mgh
29
Can a body possess energy even when it is not in motion?
Long Answers
Yes. A body at rest can possess potential energy due to its position or state.
30
Give an example showing conversion of potential energy into kinetic energy.
Long Answers
A stone dropped from a height converts potential energy into kinetic energy while falling.
31
State the energy changes in a wound-up spring while it unwinds.
Long Answers
Elastic potential energy → Kinetic energy.
D Section
Think & Answer
Conceptual reasoning and real-life situations
6 questions
32
A coolie is moving with luggage on his head. Does he perform work against gravity?
Think & Answer
No. The displacement is horizontal while gravity acts vertically downward. Hence no work is done against gravity.
33
The moon revolves around the Earth. How much work is done by gravity?
Think & Answer
Zero. Gravitational force acts towards the centre while displacement is perpendicular to it.
34
Give reasons for the following statements.
Think & Answer
(a) No work is done in pushing a wall because the wall does not move.
(b) Hammer drives a nail into wood because work is done when the nail moves.
(c) Horse has more kinetic energy than a dog at the same speed because it has greater mass.
(d) The teacher moving around the class is doing work, but not on the child because the child does not move.
35
Name the form of energy to which potential energy can change.
Think & Answer
Kinetic energy.
36
Is it practically possible to convert one form of energy completely into another useful form?
Think & Answer
No. Some energy is always lost as heat, sound, etc.
37
Standing on an escalator, is a person doing work?
Think & Answer
No. The escalator does the work; the person is not applying force to produce the motion.
E Section
Numericals
Worked calculations with formulas
12 questions
38
1. A force of 30 N acts on a body and moves it through a distance of 5 m in the direction of force. Calculate the work done.
Numerical solution
Given: Force, F = 30 N | Distance, d = 5 m Formula:W = F × d Calculation: W = 30 × 5 = 150 J Answer: Work done = 150 J
39
2. A man lifts a mass of 20 kg to a height of 2.5 m. Take g = 10 N kg ¹. Find the work done.
Numerical solution
Given: Mass, m = 20 kg | Height, h = 2.5 m | g = 10 N kg ¹ Formula:Work done = mgh Calculation: W = 20 × 10 × 2.5 = 500 J Answer: Work done = 500 J
40
3. A body acted upon by a force of 10 kgf moves through a distance of 0.5 m. Take 1 kgf = 10 N.
Numerical solution
Given: Force = 10 kgf = 100 N | Distance = 0.5 m Formula:W = F × d Calculation: W = 100 × 0.5 = 50 J Answer: Work done = 50 J
41
4. Two bodies of same mass are placed at heights h and 2h. Compare their gravitational potential energies.
Numerical solution
Formula:P.E. = mgh
For first body: P.E. ¹ = mgh
For second body: P.E. ² = mg(2h) = 2mgh Ratio: P.E. ¹ : P.E. ² = mgh : 2mgh = 1 : 2 Answer: 1 : 2
42
5. Find the gravitational potential energy of a 2.5 kg mass kept at a height of 15 m. Take g = 10 N kg ¹.
Numerical solution
Given: m = 2.5 kg | h = 15 m | g = 10 N kg ¹ Formula:P.E. = mgh Calculation: P.E. = 2.5 × 10 × 15 = 375 J Answer: Potential Energy = 375 J
43
6. The potential energy stored in a box of weight 150 kgf is 1.5 × 10⁴ J. Find the height.
Numerical solution
Given: Weight = 150 kgf = 150 × 10 = 1500 N | P.E. = 1.5 × 10⁴ J Formula:P.E. = Weight × Height Calculation:
1.5 × 10⁴ = 1500 × h
h = (1.5 × 10⁴) / 1500
h = 10 m Answer: Height = 10 m
44
7. The potential energy of a body of mass 0.5 kg increases by 100 J when taken to the top of a tower. Find the height. Take g = 10 N kg ¹.
Numerical solution
Given: m = 0.5 kg | Increase in P.E. = 100 J | g = 10 N kg ¹ Formula:P.E. = mgh Calculation:
100 = 0.5 × 10 × h
100 = 5h
h = 20 m Answer: Height of tower = 20 m
45
8. A body of mass 60 kg moves with a speed of 50 m s ¹. Find its kinetic energy.
Numerical solution
Given: m = 60 kg | v = 50 m s ¹ Formula:K.E. = ½ mv² Calculation:
K.E. = ½ × 60 × (50)²
= 30 × 2500 = 75,000 J = 7.5 × 10⁴ J Answer: Kinetic Energy = 7.5 × 10⁴ J
46
9. A truck of mass 1000 kg increases its speed from 36 km h ¹ to 72 km h ¹. Find the increase in kinetic energy.
Numerical solution
Given: m = 1000 kg
u = 36 km h ¹ = 10 m s ¹
v = 72 km h ¹ = 20 m s ¹ Formula:Increase in K.E. = ½m(v² − u²) Calculation:
= ½ × 1000 × (20² − 10²)
= 500 × (400 − 100)
= 500 × 300 = 1.5 × 10⁵ J Answer: Increase in kinetic energy = 1.5 × 10⁵ J
47
10. A car moves at 15 km h ¹ and another identical car at 30 km h ¹. Compare their kinetic energies.
Numerical solution
Since masses are the same: K.E. ∝ v²
K.E. ¹ : K.E. ² = (15)² : (30)²
= 225 : 900 = 1 : 4 Answer: 1 : 4
48
11. A pump raises water by spending 4 × 10⁵ J of energy in 10 s. Find its power.
Numerical solution
Given: Work done = 4 × 10⁵ J | Time = 10 s Formula:Power = Work Done / Time Calculation: P = (4 × 10⁵) / 10 = 4 × 10⁴ W Answer: Power = 4 × 10⁴ W
49
12. It takes 20 s for girl A and 15 s for girl B to climb the same stairs. Compare (i) work done, (ii) power spent.
Numerical solution
(i) Work Done: Both climb the same stairs. Since W = mgh, assuming both have the same weight and height, the work done is the same. Answer: Work done ratio = 1 : 1
(ii) Power Spent:Power = Work / Time
Power ratio = W/20 : W/15 = 15 : 20 = 3 : 4 Answer: Power ratio = 3 : 4
F Section
Case Study
Apply work and energy to a practical situation
3 questions
50
(i) Did Shefali perform any work on the heavier boxes?
Case Study
No. Reason: Although Shefali applied force, the heavier boxes did not move. Since displacement is zero: Work Done = Force × Displacement = Force × 0 = 0
Therefore, no work was done on the heavier boxes.
51
(ii) If the floor had been rough instead of smooth, how would the work done by Shefali change?
Case Study
A rough floor would produce greater friction. Shefali would have to apply more force to move the boxes. Therefore, more work would be done to move the same distance. Answer: Work done would increase.
52
(iii) Is Shefali's father doing any work? Explain.
Case Study
Yes.
He lifts the boxes vertically upward. A force is applied and the boxes move in the direction of the force.
Hence, Work Done = Force × Displacement.
Therefore, work is done by Shefali's father while lifting the boxes into the truck.
No matching question found. Try a shorter term such as “work”, “joule”, “speed” or “potential”.
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3. Gravitational potential energy is:
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Frequently Asked Questions
Energy chapter FAQs
Quick answers to the most common revision questions.
Energy is the capacity or ability of a body to do work. Its S.I. unit is the joule (J).
Potential energy is stored energy due to position or state, while kinetic energy is the energy a body possesses due to motion.
When force and displacement are in the same direction, work done is W = F × d, where F is force and d is displacement.
Gravitational potential energy is P.E. = mgh, where m is mass, g is acceleration due to gravity and h is height.
Kinetic energy becomes four times because K.E. is proportional to the square of speed.
Yes. If the body has no displacement, or if displacement is perpendicular to the force, the work done by that force is zero.
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