Worksheet Solutions: Work, Energy and Power | Physics Class 10 ICSE PDF Download

Part A — Multiple Choice Questions 

Q1. When is the work done by a force maximum?

• (a) When angle between force and displacement is 0°

• (b) When angle is 90°

• (c) When angle is 180°

• (d) When displacement is zero

Answer: (a) 0°
Explanation: Work = Force × Displacement × cosθ. For θ = 0°, cosθ = 1, so work is maximum.

Q2. The SI unit of work is:

• (a) Newton

• (b) Joule

• (c) Watt

• (d) Erg

Answer: (b) Joule
Explanation: 1 joule = Work done when a force of 1 newton displaces a body by 1 metre in its direction.

Q3. Which of the following is a commercial unit of energy?

• (a) Erg

• (b) Watt

• (c) Kilowatt hour

• (d) Joule

Answer: (c) Kilowatt hour
Explanation: Electricity consumption is measured in kilowatt hours (kWh).

Q4. The kinetic energy of a body depends on:

• (a) Mass only

• (b) Velocity only

• (c) Both mass and velocity

• (d) Acceleration only

Answer: (c) Both mass and velocity
Explanation: Kinetic energy = ½ × mass × velocity².

Q5. According to the law of conservation of energy:

• (a) Energy can be created but not destroyed

• (b) Energy can be destroyed but not created

• (c) Energy can neither be created nor destroyed

• (d) Energy is always lost as heat

Answer: (c) Energy can neither be created nor destroyed
Explanation: Energy only changes from one form to another, total remains constant.

Part B — Short Answer Questions

Q6. Define work in scientific terms. Give one condition when no work is done.
Answer: Work is said to be done when a force is applied on a body and it causes displacement in the direction of force. If displacement is zero, no work is done. Example: When you push a wall and it does not move, work done is zero.

Q7. Calculate the work done in lifting a 10 kg object vertically upward to a height of 2 m. (Take g = 10 m/s²)
Answer:
Force = Weight = mass × gravity = 10 × 10 = 100 N
Displacement = 2 m
Work = Force × Displacement = 100 × 2 = 200 J

Q8. A machine does 200 J of work in 10 seconds. Find its power.
Answer:
Power = Work ÷ Time = 200 ÷ 10 = 20 W

Q9. A man pushes a cart with a force of 50 N making an angle of 60° with the horizontal. The cart moves 10 m along the horizontal ground. Find the work done by the man.
Answer:
Work = Force × Displacement × cosθ
= 50 × 10 × cos 60°
= 500 × 0.5 = 250 J

Q10. State one difference between kinetic energy and potential energy with an example of each.
Answer:

• Kinetic energy: Energy of a body due to motion. Example: A moving car.

• Potential energy: Energy of a body due to position or configuration. Example: Water stored in a tank.

Part C — Long Answer Questions

Q11. Explain the difference between positive work, negative work and zero work with examples.
Answer:

1. Positive Work

• When force and displacement of an object are in the same direction, the work done is positive.

• Example: When you push a trolley forward and it moves in the same direction, you are doing positive work.

2. Negative Work

• When force and displacement are in opposite directions, the work done is negative.

• Example: Friction does negative work on a moving car because friction acts opposite to the motion.

3. Zero Work

• When there is no displacement, or the force is perpendicular to displacement, the work done is zero.

• Example: Holding a heavy suitcase without moving it — force is applied, but no displacement, so work is zero.

• Example 2: In circular motion, centripetal force acts at right angles to displacement, so work done is zero.

Q12. A ball of mass 1 kg is thrown vertically upward with a speed of 10 m/s. Calculate:
(a) its initial kinetic energy
(b) the maximum height it reaches
(c) its potential energy at the highest point
(Take g = 10 m/s²)

Step 1: Initial Kinetic Energy (KE)
Formula: KE = ½ × mass × velocity²
= ½ × 1 × (10)²
= ½ × 100
= 50 J

Step 2: Maximum Height Reached
At the top, all KE is converted into Potential Energy (PE).
So, PE at top = KE at bottom = 50 J
PE = mass × g × height
50 = 1 × 10 × height
Height = 50 ÷ 10 = 5 m

Step 3: Potential Energy at the Highest Point
At the highest point, KE = 0 and PE = maximum.
PE = m × g × h = 1 × 10 × 5 = 50 J

Q13. State and explain the law of conservation of energy with the example of a simple pendulum.
Answer:

Statement:
Energy can neither be created nor destroyed; it can only be changed from one form to another. The total energy of an isolated system remains constant, although it may appear in different forms like kinetic energy, potential energy, heat, light, etc.

Explanation of simple pendulum

A simple pendulum consists of a bob suspended from a fixed point by a string. When displaced from its mean (rest) position, it oscillates to and fro.

1. At the extreme position (maximum displacement):

   • The bob is raised to a height.

   • It has maximum potential energy (mgh).

   • Kinetic energy = 0 (since velocity = 0).

2. At the mean position (lowest point):

   • The bob is at its lowest height.

   • Potential energy = 0 (taking reference at mean position).

   • The bob has maximum kinetic energy due to maximum velocity.

3. At an intermediate position:

   • The bob has both potential energy and kinetic energy.

   • The sum of both remains constant.

Thus, during oscillation, energy continuously changes from potential to kinetic and back to potential, but the total mechanical energy (PE + KE) remains constant (ignoring air resistance).