Gravitational potential energy
Potential energy is stored energy. The most common kind at this level is gravitational potential energy, the energy an object has because of its height above the ground. It is calculated with PE = m × g × h, where m is the mass in kilograms, g is the gravitational field strength (about 10 N/kg on Earth) and h is the height in metres. A heavier object, or one lifted higher, stores more potential energy.
Key idea
Potential energy: PE = m g h. Kinetic energy: KE = ½ m v². Both are measured in joules (J).
Kinetic energy
Kinetic energy is the energy an object has because it is moving. It is found from KE = ½ × m × v², where v is the speed in metres per second. Because the speed is squared, doubling the speed makes the kinetic energy four times larger. This is why a fast-moving car is so dangerous in a crash.
Example
A 2 kg ball is held 5 m up. PE = 2 × 10 × 5 = 100 J. As it falls, this becomes kinetic energy. Just before it lands, KE ≈ 100 J, giving a speed of about 10 m/s.
Changing from one to the other
By the principle of conservation of energy, energy is not lost, only changed. As an object falls, its potential energy decreases while its kinetic energy increases. At the highest point the potential energy is greatest and the kinetic energy is least; at the lowest point the kinetic energy is greatest.
Remember
- PE depends on mass and height; KE depends on mass and speed.
- Kinetic energy grows with the square of the speed.
- Falling: PE changes into KE, total energy is conserved.