Kinetic, gravitational and elastic energy

Energy is measured in joules (J). A moving object has kinetic energy: Eₖ = ½ m v², with mass in kg and speed in m/s. A raised object gains gravitational potential energy: Eₚ = m g h, where g is about 9.8 N/kg on Earth and h is the height gained in metres. A stretched spring stores elastic potential energy: Eₑ = ½ k e², where k is the spring constant and e the extension.

In a closed system the total energy stays the same: energy is transferred between stores, never created or destroyed. If a ball falls with no air resistance, the Eₚ it loses equals the Eₖ it gains. Work done is energy transferred by a force: W = F × s, where s is the distance moved in the direction of the force.