Pulling a rope: an example of a force 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.
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.
Liquids and gases exert pressure on you. The weight of air on your body is balanced by the pressure exerted by the water inside your body, otherwise you would be crushed. Air pressure decreases the higher up you are as there is less and less air. The deeper you go in the water, the greater the pressure. People cannot dive deeper than about 120 metres (400 feet) as the water pressure would crush them.
Force is measured in units called newtons after scientist Isaac Newton. Pressure is measured in newtons per square metre.
A plane flies due to a balance of forcesForces always act in pairs. When both forces are equal, an object will stay still or will move at a constant speed in one direction. When one force is greater than the other, the forces are unbalanced and the object will change direction or speed. When several forces act on an object, the overall effect is called the resultant force. For example, lift, thrust, gravity and drag all act on an aeroplane, while its resultant force is a forward motion.
Centrifugal and centripetal forces
When we sit on a fairground ride like a Chair-o-Plane, we can feel a force pushing us quite strongly outwards. We have to hold tight to stay on! This force is called the centrifugal force. At the same time there is another force at work, keeping us moving in a circle around the roundabout. This force is called the centripetal force. It is these two balancing forces that keep the Earth and other planets travelling in orbit around the Sun.
All the forces in the Universe are based on four fundamental forces. The first two, the strong and weak nuclear forces, act only at very short distances. They are responsible for holding together the subatomic particles inside the nucleus of an atom. The third force, the electromagnetic force, acts between electrically charged particles. This is the force that attracts electrons to an atomic nucleus to form atoms, the building blocks of all matter. The fourth force is the gravitational force, or gravity, which acts between masses.
Consultant: Dave Hawksett