Concise Physics Class 8 - Energy (Questions & Answers)

Energy – Concise Physics Class 8 Solutions

B. Short Answer Type Questions

Define work.

Work is said to be done when a force applied on a body produces displacement in the direction of the force.

When does a force perform work?

A force performs work when it causes a body to move in the direction of the force.

State two conditions when no work is done by a force.

• When no force acts on the body.
• When the body does not move despite the force acting on it.

In which of the following cases is work being done?

• (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.

Write the expression for work done by a force.

W = F × d

where, W = work done, F = force applied, d = displacement in the direction of force.

State the S.I. unit of work and define it.

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.

State two factors on which the work done on a body depends.

• Magnitude of force applied.
• Distance moved in the direction of the force.

Define the term energy.

Energy is the capacity (ability) to do work.

State the S.I. unit of energy.

joule (J).

Define 1 joule of energy.

One joule of energy is the energy possessed by a body capable of doing one joule of work.

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.

What are the two kinds of mechanical energy?

• Potential Energy
• Kinetic Energy

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)

Give one example of a body that has potential energy.

• (a) Due to position at a height: Water stored in a tank.
• (b) Due to elongated stretched state: A stretched rubber band.

Which bucket has greater potential energy?

The bucket kept on the second floor has greater potential energy because it is at a greater height.

Write the expression for gravitational potential energy.

P.E. = mgh

where, m = mass of body, g = acceleration due to gravity, h = height above the ground.

Define kinetic energy. Give one example.

Kinetic energy is the energy possessed by a body due to its motion.
Example: A moving car.

State two factors on which kinetic energy depends.

• Mass of the body.
• Speed (velocity) of the body.

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.

A cyclist doubles his speed. How will his kinetic energy change?

Kinetic energy becomes four times because K.E. ∝ v².

Write the expression for kinetic energy.

K.E. = ½ mv²

where, m = mass of body, v = speed of body.

A ball of mass m is moving with speed v. What is its kinetic energy?

K.E. = ½ mv²

Name the form of energy stored in a wound-up spring of a watch.

Elastic potential energy.

Name the type of energy possessed by:

• (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

State the energy changes:

• (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. Long Answer Type Questions

State two factors on which potential energy depends.

• Mass of the body.
• Height of the body above the ground.

Two bodies A and B have masses 10 kg and 20 kg at the same height. Which has greater potential energy?

Body B has greater potential energy because P.E. = mgh and its mass is greater.

A body of mass m is moved from ground to height h. Calculate force needed, work done, and energy stored.

• (a) Force needed = mg
• (b) Work done = Force × Distance = mg × h = mgh
• (c) Potential energy stored = mgh

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.

Give an example showing conversion of potential energy into kinetic energy.

A stone dropped from a height converts potential energy into kinetic energy while falling.

State the energy changes in a wound-up spring while it unwinds.

Elastic potential energy → Kinetic energy.

D. Think and Answer

A coolie is moving with luggage on his head. Does he perform work against gravity?

No. The displacement is horizontal while gravity acts vertically downward. Hence no work is done against gravity.

The moon revolves around the Earth. How much work is done by gravity?

Zero. Gravitational force acts towards the centre while displacement is perpendicular to it.

Give reasons for the following statements.

• (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.

Name the form of energy to which potential energy can change.

Kinetic energy.

Is it practically possible to convert one form of energy completely into another useful form?

No. Some energy is always lost as heat, sound, etc.

Standing on an escalator, is a person doing work?

No. The escalator does the work; the person is not applying force to produce the motion.

E. Numericals – Step-by-Step Solutions

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.

Given: Force, F = 30 N | Distance, d = 5 m
Formula: W = F × d
Calculation: W = 30 × 5 = 150 J
Answer: Work done = 150 J

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.

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

3. A body acted upon by a force of 10 kgf moves through a distance of 0.5 m. Take 1 kgf = 10 N.

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

4. Two bodies of same mass are placed at heights h and 2h. Compare their gravitational potential energies.

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

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.

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

6. The potential energy stored in a box of weight 150 kgf is 1.5 × 10⁴ J. Find the height.

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

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.

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

8. A body of mass 60 kg moves with a speed of 50 m s ¹. Find its kinetic energy.

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

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.

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

10. A car moves at 15 km h ¹ and another identical car at 30 km h ¹. Compare their kinetic energies.

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

11. A pump raises water by spending 4 × 10⁵ J of energy in 10 s. Find its power.

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

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.

(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. Case Study Based Question

(i) Did Shefali perform any work on the heavier boxes?

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.

(ii) If the floor had been rough instead of smooth, how would the work done by Shefali change?

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.

(iii) Is Shefali's father doing any work? Explain.

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.