7.2 | Potential Energy and Kinetic Energy
Gravitational potential 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:
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
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:
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
Kinetic energy
Kinetic energy is the energy possessed by a moving object.
Formula:
Examples: