Frictional Force (or Friction)
A ball moving on the ground slows down gradually and stops after covering some distance. We know that a force is required to stop a moving body. This means that a force is exerted by the ground on the moving ball which opposes its motion and brings it to a stop. This force which opposes the motion of ball on the ground in known as frictional force. We can now define the frictional force as follows : The force which always opposes the motion of one body over another body is called frictional force (or friction). The frictional force acts between the two surfaces which are in contact with each other. A ball moving on the ground slows down and then stops due to frictional force between the ball and the ground. In this case the two surfaces in contact are the surface of the ground and the surface of the ball. Frictional force is also known as ‘force of friction’ or just ‘friction’. We will now give some more examples from everyday life where frictional force is involved.
If we stop pedaling a running bicycle, it slows down gradually and stops after covering some distance. The bicycle moving on the road slows down and finally comes to a stop due to the frictional force between the tyres of the bicycle and the road. This frictional force opposes the motion of bicycle and brings it to a stop. In this case, the two surfaces in contact are the surface of boat and the surface of water.
Since muscular force can be applied to an object only when our body (or body of an animal) is in contact with the object, therefore, muscular force is a contact force. For example, when we lift a book from a table by applying muscular force, then our hand is in contact with the book. We cannot lift the book from table without touching it. Our contact (or that of an animal) with an object for applying muscular force can also be through a stick or a piece of rope, etc. For example, when we hit a hockey ball, we apply muscular force to ball through the hockey stick. And when we draw a bucket of water from a well, then we apply the muscular force through a rope (tied to the bucket).
The frictional force(or friction) always acts on all moving objects, the direction of which is always opposite to the direction of motion. Since frictional force arises only when the surfaces of two objects are in touch with each other, therefore, frictional force is an example of a contact force.
The animals like an ox, camel and horse provide the force of their muscles and help the man in doing harder jobs. For example, ox pulls the plough by using its muscular force. Similarly, a horse and camel exert their muscular force in pulling the carts. An elephant pulls heavy logs of wood by using the muscular force. And in arctic regions (snow bound regions), reindeer’s pull the sledges (wheel less vehicles) by using their muscular force.
Please note that it is not necessary that the agent applying a force on an object should always be in contact with it. There are some forces which act on an object even from a distance. We will now discuss such non contact forces.