According to Newton's first law of motion every body continues to be in its state of rest or uniform motion along a straight line unless an external force acts on it. This means that a ball moving on a horizontal surface should continue to move indefinitely as no force is acting on it. But practically this does not happen. After sometime the speed of the ball decreases and eventually it comes to rest. Now the question is what stopped the ball? The ball stops because the surface opposes the motion of the ball i.e., the ball experiences a force, which opposes its motion. This force is called the force of friction or simply friction.
The force of friction or simply friction is that force which opposes the motion of an object over another object in contact with it.
The force of friction between two surfaces will retard the motion of the object and finally will bring it to rest. An external force, which is equal and opposite to the force of friction, must continuously act on the moving object in order to maintain the uniform motion.
The maximum value of the force of friction, which comes into play when a body just begins to move over the surface of another body is known as the limiting friction.When there is no relative motion between the two surfaces in contact, then the friction is a static friction.
Friction is capable of adjusting itself not only with regard to its magnitude but also the direction of the applied force.The force required to keep the body in steady motion after the motion has once started is called the sliding friction. The sliding friction is smaller than the limiting friction.
It is a matter of common experience that it is easier to roll a body than to slide it along the ground. That is because rolling friction is always less than sliding friction. It is probably due to this reason primitive man started using circular wheels for the carriages. In modern applications, the sliding friction between the shaft and the wheel is changed into rolling friction by using ball bearings.Liquids and gases also exert friction. However, friction offered by them is less compared to that offered by solid surfaces.
The best example to demonstrate the friction offered by air is the meteor shower. Each time a comet passes the Sun it loses some of its gas and ultimately only the dust particles and bits of rocks are left in space. When these particles enter earth's atmosphere, they burn due to the heat produced by air resistance and produce a shower of meteors or shooting stars.The friction due to water is much less than that between solid surfaces and hence it is difficult to stop ships or boats. To stop a ship the engine is fired in the opposite direction.
Now let us see how the force of friction can be increased and decreased.
Friction can be Decreased by the following methods:
Lubricants like oil or grease can reduce friction.
The lubricants form a thin film between the moving parts and fill the tiny pores and depressions on the surfaces.
This makes the surfaces smooth and reduces the friction.
Oil Grease Graphite Powder
As rolling friction is less than sliding friction, ball bearings and roller bearings are also used for decreasing friction.Small balls or rollers made of steel are placed between the moving parts.
When steel slides over an alloy of lead, the friction is less than when steel slides on steel.Hence bearings are packed with an alloy of lead.
For example, a hovercraft can travel over rough land, swamp or sea. As the moving object does not come in contact with the other solid surface, the frictional forces are reduced.
The shape of many objects moving through air or water is designed in such a way that friction between the objects and the medium through which they move is minimised.
Aeroplanes, boats and paper rockets are examples of streamlined objects.
Aeroplane Boat Paper Rockets
- By polishing the surfaces the friction between the moving surfaces can be reduced.