The energy in a moving object is stored in its kinetic store. It is found with Ek = 0.5 × m × v2, where Ek is in joules (J), mass m is in kilograms (kg) and speed v is in metres per second (m/s). The speed is squared, so doubling the speed makes the kinetic energy four times bigger. For example, a 2 kg trolley moving at 3 m/s has Ek = 0.5 × 2 × 32 = 9 J.
A stretched spring stores energy in its elastic potential store. It is found with Ee = 0.5 × k × e2, where k is the spring constant in newtons per metre (N/m) and e is the extension in metres (m). This only works while the spring has not gone past its limit of proportionality. For example, a spring with k = 50 N/m stretched by 0.2 m has an extension squared of 0.04 m2, so Ee = 0.5 × 50 × 0.04 = 1 J.
An object raised above the ground gains energy in its gravitational potential store. It is found with Ep = m × g × h, where g is the gravitational field strength in newtons per kilogram (N/kg) and h is the height in metres. In a calculation the value of g is given, for example 10 N/kg. A 3 kg object raised by 2 m gains 3 × 10 × 2 = 60 J. If it falls freely, this energy is transferred to its kinetic store.
To investigate this transfer, you can release a trolley or a mass from different heights and measure its speed at the bottom. A greater height gives a greater gravitational potential energy to transfer, so a higher speed.