Energy – Short Answer Type Questions
Concise Physics ICSE Class 8
1. Define Work.
Work is said to be done when a force applied on a body produces displacement in the direction of the force.
Work is done only when both force and displacement act in the same direction.
2. When does a force perform work?
A force performs work when it causes a body to move in the direction of the force.
Force alone cannot do work. Movement must occur.
3. State two conditions when no work is done by a force.
- No force acts on the body.
- The body does not move despite the force acting on it.
Without force or without displacement, work done is zero.
4. In which of the following cases is work being done?
- Boy pushing heavy rock – No work (if rock does not move)
- Boy climbing stairs – Work is done
- Coolie standing with a box – No work
- Girl moving on the road – Work is done
Work is done only when displacement occurs.
5. Write the expression for work done.
W = F × d
Where,
- W = Work Done
- F = Force
- d = Displacement
Work equals force multiplied by displacement.
6. State the SI unit of work and define it.
SI 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.
1 J = 1 N × 1 m
7. State two factors on which work done depends.
- Magnitude of force.
- Distance moved in the direction of force.
Greater force or greater displacement produces more work.
8. Define Energy.
Energy is the capacity (ability) to do work.
Energy enables an object to perform work.
9. State the S.I. unit of energy.
The S.I. unit of energy is joule (J).
Both work and energy are measured in joules (J).
10. Define 1 joule of energy.
One joule of energy is the energy possessed by a body capable of doing one joule of work.
1 joule of energy enables a body to perform 1 joule of work.
11. How is work related to energy?
Energy is measured by the amount of work done. Work done on a body equals the energy gained by it.
Work transfers energy from one body to another.
12. What are the two kinds of mechanical energy?
- Potential Energy
- Kinetic Energy
Mechanical energy is of two types: Potential Energy and Kinetic Energy.
13. What is potential energy? State its unit.
Potential energy is the energy possessed by a body due to its position or state.
Unit: joule (J)
A raised or stretched object stores potential energy.
14. Give one example of a body that has potential energy.
- Water stored in an overhead tank.
- A stretched rubber band.
Potential energy may be due to height or stretching.
15. Which bucket has greater potential energy?
The bucket kept on the second floor has greater potential energy because it is at a greater height.
Potential energy increases with height.
16. Write the expression for gravitational potential energy.
P.E. = mgh
Where,
- m = mass of the body
- g = acceleration due to gravity
- h = height above the ground
Gravitational potential energy depends on mass, gravity and height.
17. Define kinetic energy. Give one example.
Kinetic energy is the energy possessed by a body due to its motion.
Example: A moving car.
Any moving object possesses kinetic energy.
18. State two factors on which kinetic energy depends.
- Mass of the body.
- Speed (velocity) of the body.
Kinetic energy increases with both mass and the square of speed.
19. Two toy cars A and B of masses 200 g and 500 g move with the same speed. Which has greater kinetic energy?
Car B (500 g) has greater kinetic energy because kinetic energy is directly proportional to mass.
For the same speed, the heavier object has greater kinetic energy.
20. A cyclist doubles his speed. How will his kinetic energy change?
The kinetic energy becomes four times because K.E. ∝ v².
If speed doubles, kinetic energy becomes 2² = 4 times.
21. Write the expression for kinetic energy.
K.E. = ½ mv²
Where:
- m = mass of the body
- v = speed of the body
This is the standard formula for kinetic energy.
22. A ball of mass m is moving with speed v. What is its kinetic energy?
K.E. = ½ mv²
Substitute the values of mass and speed into the formula.
23. Name the form of energy stored in a wound-up spring of a watch.
Elastic Potential Energy.
A stretched or compressed spring stores elastic potential energy.
24. Name the type of energy possessed by the following.
| Object | Energy |
|---|---|
| Moving cricket ball | Kinetic Energy |
| Stone at rest on top of a building | Potential Energy |
| Compressed spring | Potential Energy |
| Moving bus | Kinetic Energy |
| Bullet fired from a gun | Kinetic Energy |
| Water flowing in a river | Kinetic Energy |
| Stretched rubber band | Potential Energy |
Moving objects possess kinetic energy, while raised, stretched or compressed objects possess potential energy.
C. Long Answer Type Questions
1. State two factors on which potential energy depends.
- Mass of the body.
- Height of the body above the ground.
2. Two bodies A and B have masses 10 kg and 20 kg at the same height. Which has greater potential energy?
Gravitational potential energy is given by P.E. = mgh.
Since both bodies are at the same height and experience the same value of g, potential energy depends only on mass. Therefore, Body B (20 kg) has greater potential energy.
3. A body of mass m is moved from the ground to a height h.
Force = mg
(b) Work doneWork Done = Force × Distance = mg × h = mgh
(c) Potential energy storedPotential Energy = mgh
4. Can a body possess energy even when it is not in motion?
Yes. A body at rest can possess potential energy due to its position or state.
5. Give an example showing conversion of potential energy into kinetic energy.
A stone dropped from a height converts its potential energy into kinetic energy while falling.
6. State the energy changes in a wound-up spring while it unwinds.
Elastic Potential Energy → Kinetic Energy
D. Think and Answer
1. A coolie is moving with luggage on his head. Does he perform work against gravity?
Answer:
No. The displacement of the luggage is horizontal, whereas the force of gravity acts vertically downward. Since displacement is perpendicular to gravity, no work is done against gravity.
2. The Moon revolves around the Earth. How much work is done by gravity?
Answer:
Zero. Gravitational force acts towards the centre of the Earth, while the Moon's instantaneous displacement is perpendicular to the force. Hence, no work is done by gravity.
3. Give reasons for the following.
(a) No work is done in pushing a wall.
The wall does not move, so displacement is zero. Therefore, no work is done.
(b) A hammer drives a nail into wood.
The hammer applies force and the nail moves into the wood. Hence, work is done.
(c) A horse has more kinetic energy than a dog at the same speed.
Kinetic energy depends on mass. Since a horse has greater mass, it possesses more kinetic energy.
(d) A teacher moving around the classroom is doing work, but not on the child.
The teacher moves and does work on herself. The child does not move, so no work is done on the child.
4. Name the form of energy into which potential energy can change.
Answer:
Potential energy can be converted into kinetic energy.
5. Is it practically possible to convert one form of energy completely into another useful form?
Answer:
No. In every energy conversion, some energy is always lost as heat, sound or other forms. Therefore, complete conversion into useful energy is not practically possible.
6. Standing on an escalator, is a person doing work?
Answer:
No. The escalator moves the person without the person applying a force to produce the motion. Therefore, the person does not do any work.
