Define kinetic energy and potential energy. A 0.5 kg stone is held 4 m above the ground. Find its potential energy, and its kinetic energy just before it hits the ground when dropped. (g = 9.8 m s⁻²)

1. 1 · Define KE. Kinetic energy is the energy of a body due to its motion: KE = ½mv².
2. 2 · Define PE. Potential energy is the energy of a body due to its position or shape: PE = mgh.
3. 3 · Find PE at the top. PE = mgh = 0.5 × 9.8 × 4 = 19.6 J.
4. 4 · Find KE at the bottom. As the stone falls, its PE changes into KE. Ignoring air resistance, KE just before landing = 19.6 J.
5. 5 · Close. So the stone has 19.6 J of PE at the top and 19.6 J of KE at the bottom.

Scoring tip: Write both formulas before substituting. Say 'ignoring air resistance' when you use the energy change.