7.2 | Potential Energy and Kinetic Energy

• Gravitational potential energy is the work done to lift an object to a height, h from the Earth’s surface.

• Gravitational potential energy = Work done

= Force (F) × Displacement (s)

= mgh

• Objects that are at a height from the Earth’s surface have the gravitational potential energy.

• Examples:

1. The photo on the right shows a lift in a shopping centre that carried a 500 kg load to a height of 20 m in 20 s. (g is estimated as 10 m s–2)

(a) What is the word done by the lift?

Work done = mgh

= 500 × 10 × 20

= 100 000 J

(b) What is the gravitational potential energy of the lift at the height of 20 m?

Gravitational potential energy = work done = 100 000 J

(c) Calculate the power of the lift.

Power of the lift = 100 000 J

20 s

= 5 000 watt

• Elastic potential energy is the work done to compress or stretch an elastic material with displacement x from the equilibrium position.

• A compressed or stretch spring possessed elastic potential energy.

• Elastic potential energy = Work done = Average force (N) × Displacement (m) = 1/2 Fx

• Examples:

1. The diagram on the right shows a compressed spring. The original length of the spring is 15 cm. The length of the spring becomes 10 cm when a force of 20 N is applied.

(a) What is the compression distance, x of the spring?

Compression distance, x = 15 cm – 10 cm

= 5 cm = 0.05 m

(b) Calculate the elastic potential energy possessed by the compressed spring.

Elastic potential energy = 1/2 Fx

= 1 2 × 20 N × 0.05 m

= 0.5 J