Chapter 2 · ICSE Class 10 Physics (Selina Concise)

Work, Energy and Power

A falling ball trades stored energy for motion, and the total never changes. Learn how work, energy and power are measured.

⚡ Chapter 2

Energy of a freely falling body A 2 kilogram ball falls from a height of 5 metres. At the top it has 100 joules of potential energy and no kinetic energy. Halfway down it has 50 joules of each. At the ground it has 100 joules of kinetic energy and no potential energy. The total stays 100 joules. Ball of mass 2 kg falling from 5 m (g = 10 m/s²) 5 m 2.5 m 0 PE 100 J KE 0 PE 50 J KE 50 J PE 0 KE 100 J PE + KE = 100 J at every height
2.1

Work

Work is done when a force moves a body in the direction of the force: work = force × displacement. The SI unit is the joule (J), where 1 J = 1 N × 1 m. Work done against gravity in lifting a body of mass m through a height h is mgh. (Take g = 10 m/s².)

1

A force of 20 N moves a body 3 m in its own direction. How much work is done?

20 × 3 = 60 J

2

How much work is done in lifting a 5 kg load through 2 m?

mgh = 5 × 10 × 2 = 100 J

2.2

Kinetic and Potential Energy

Energy is the capacity to do work, and it is also measured in joules. Kinetic energy is the energy of motion, KE = ½mv². Potential energy due to height is PE = mgh. By the law of conservation of energy, energy is never created or destroyed, only changed from one form to another, so for a freely falling body KE + PE stays constant.

1

Find the kinetic energy of a 2 kg body moving at 3 m/s.

½ × 2 × 3² = 9 J

2

A 2 kg ball falls from 5 m. At what speed does it reach the ground, and what is its kinetic energy then?

From mgh = ½mv², v = √(2gh) = √100 = 10 m/s, and KE = ½ × 2 × 10² = 100 J, equal to the PE at the top

3

If the speed of a moving body is doubled, what happens to its kinetic energy?

It becomes four times as great, since KE depends on v²

2.3

Power

Power is the rate of doing work: P = W / t. Its SI unit is the watt (W), where 1 W = 1 J/s. A larger unit is the kilowatt (1 kW = 1000 W), and 1 horsepower = 746 W. Electrical energy is billed in kilowatt-hours, where 1 kWh = 3.6 × 10⁶ J.

1

A machine does 600 J of work in 20 s. What is its power?

600 ÷ 20 = 30 W

2

A motor lifts a 100 kg load through 15 m in 20 s. Find its power.

P = mgh / t = 100 × 10 × 15 ÷ 20 = 750 W

3

How many kilowatt-hours of energy does a 2 kW heater use in 3 hours?

2 × 3 = 6 kWh

Practice

Exercises

1. State the SI unit of work.

2. How much work is done when a force of 20 N moves a body 3 m along the force?

3. Write the expression for kinetic energy.

4. Write the expression for the potential energy of a body of mass m at height h.

5. Find the kinetic energy of a 2 kg body moving at 3 m/s.

6. State the law of conservation of energy.

7. A ball is dropped from a height. At what stage is its kinetic energy greatest?

8. Define power and state its SI unit.

9. A motor lifts a 100 kg load through 15 m in 20 s. What is its power? (g = 10 m/s²)

10. How much energy in kilowatt-hours does a 2 kW heater use in 3 hours?

Next chapter — Machines →