The Online Library of Knowledge

# Forces

Anything that pushes or pulls an object is called a force. A breath of wind, a car engine, a magnet and an elastic band are all examples of things that can produce a force. A force may make the object move in a certain way (motion) or change its shape (deform). Gravity is a very important force. You can see it at work if you let a pen drop. The pen is being pulled towards a very large object—the Earth itself.

## Pressure

Hammer a nail into wood, and the force of the hammer makes it push down into the wood. This is a force causing motion. The wood pushes back with its own resisting force. As the nail goes deeper, the resisting force of the wood increases. It may eventually equal the force of the hammer hitting the nail. At this stage the nail stops moving. The next blow may bend the nail. This is a force causing deformation.

The amount of force for a certain area is called pressure. A nail goes into wood easily because the force hammering it is concentrated into the tiny area of the nail's sharp point, giving high pressure. The same hammer blow might not make a blunt nail enter the wood because the force is spread out over a larger area: the pressure is lower.

A jacana, a bird with long toes and claws, can walk over lilypads resting on the water's surface without sinking in because its weight (the downward force of its mass) is spread over a wide area.