Potential energy

If an object is in a state where it could do work if released then we say that it has POTENTIAL ENERGY (sometimes written as p.e.). For this reason it can be called stored energy. You can think of potential energy that is "locked away" in an object and that can be used to do something when it is "let out". Just think of the energy released if a dam bursts or if the Earth's crust splits or bends as happens in an earthquake. So, an object held above the ground has GRAVITATIONAL potential energy (G.P.E). A simpler word for this would be POSITION ENERGY. We will look at gravitational potential energy a little more carefully. Now: Work done = energy gained = force x distance moved. Therefore to move a mass m up to a height (h) above the ground you will need to move a force F (= mg) through a distance h and so this needs mgh joules of energy. So if it takes that amount of energy to get it up then its gravitational potential energy is also mgh because it will release that energy when it falls to the ground (g is the gravitational acceleration = 10 N/kg on the Earth).


When you lift up a 1 kg mass a distance of 1m you change 10 J of chemical energy in your body to 10 J of potential energy (ignoring heat for the moment). If you climb a steep mountain slope it is easier if you do it in a zig zag path. The gravitational potential energy that you gain is the same as if you had gone straight up but you gain it more slowly. The same also applies to driving a car up a mountain road – this is why they are built with so many bends, the gain height slowly. (There is some more information in the file called TRAINS See: 14-16/mechanics/Forces in motion/Text/Trains)


You should realise that it does not matter which way the mass falls to the ground in example 1 or which way the box get to the back of the lorry in example 3. The potential energy change will be the same by whatever route.