Copyright © 2007 Dorling Kindersley
If you kick a ball across a playground, it bounces and rolls on the ground’s rough surface and soon comes to a halt. What slows it down is friction, which is the force between a moving object and whatever it touches. Cars travel faster if they are STREAMLINED to reduce a type of friction called air resistance. Friction can sometimes be helpful. Without friction between the tyres and the road, cars would not have enough grip to go around corners.
Slippery substances such as oil reduce the friction between two surfaces. This is known as lubrication. Machinery has to be lubricated to prevent its moving parts from wearing out due to friction. Most machines are oiled or greased when they are made and are lubricated from time to time as they are used.
When objects move, the air around them generates a type of friction called air resistance, or drag, that slows them down. Fast-moving objects such as cars, trains, and aeroplanes are all streamlined – designed with curved and sloping surfaces to cut through the air and reduce drag. This helps them to move faster and use less fuel. Boats can be streamlined too, to reduce water resistance.