One of the best-known forms of energy is called mechanical energy. This is the energy of bodies in motion or bodies that are able to move. There are two forms of mechanical energy: kinetic energy and potential energy.

Kinetic energy

Imagine a heavy rock on top of a hill. If you gave this rock a push, it would start rolling down the hill. This would be very dangerous. Imagine the damage such a rock could do. It could hit a person with a lot of force. It could kill someone.

A big rock, moving fast, could destroy a house. This tells us that a body in motion possesses a lot of energy because it is moving. This is called kinetic energy. The word 'kinetic' comes from a Greek word meaning ‘movement'.

The amount of kinetic energy that a body has depends on the mass of the body and the speed at which it is moving. A body moving fast has more kinetic energy than the same body moving slowly. A heavy body in motion has more kinetic energy than a less heavy body moving at the same speed.

Potential energy

Think again about the rock on top of the hill, before we gave it a push and started it rolling down the hill. The rock is not moving. Because it is at the top of a hill, it has the ability to roll down and do a lot of damage on the way-as we have seen. Before the rock starts moving, we say it has potential energy. It could move down the slope, and while it was moving it would have kinetic energy. When the rock starts to move, we could say that its potential energy is being converted into kinetic energy.

Think of a marble at the top of some steps. The marble can drop down from one step (level) to the next. Each time it drops down a step, it loses some potential energy. When it has got as far down as it can fall, it has no more potential energy.