Energy can be stored in a stretched spring or in an object lifted above the ground. The energy stored in a stretched spring is the elastic potential energy, found using E = ½ × k × e2. Here k is the spring constant in N/m and e is the extension in metres. The energy stored in a raised object is its gravitational potential energy, found using E = m × g × h, where m is the mass in kg, g is the gravitational field strength (10 N/kg) and h is the height gained in metres. Both energies are measured in joules (J).
When a force moves an object, work is done: work done = force × distance moved along the line of the force. A force in newtons multiplied by a distance in metres gives a unit called the newton-metre, and 1 newton-metre (Nm) is the same as 1 joule. The energy transferred is equal to the work done.
Example: lifting a 2 kg mass through 3 m needs a force equal to its weight, 20 N. The work done is 20 × 3 = 60 J, which is the gravitational potential energy gained. Another example: a spring with a spring constant of 200 N/m stretched by 0.1 m stores ½ × 200 × 0.12 = 1 J.