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Static and Kinetic Friction

The force of friction is the force which comes into play when one body moves over (rolls or slides on) the surface of another.

This force is

1. Tangential to the surfaces in contact and opposite to the direction of motion of the body.

2. It depends on the nature of the two surfaces.

It is due to the friction offered by the ground that we are able to walk.

Static Friction


A block is kept on a horizontal surface. A horizontal force F is applied. The block does not move. This is because the applied force balances friction. This frictional force which just balances the applied force is called static friction. It is called so because the body is still at rest.
 

If the applied force is increased, the friction also increases and reaches a maximum. Just before the body starts sliding over the surface of another body, the value of frictional force is maximum and this value is called limiting friction.

Kinetic Friction

Once the block starts sliding the frictional force decreases. Hence the force required to keep the body in steady motion is less than the force required just to start it sliding. This force of friction, which comes into play when a body is in a steady state of motion over another surface, is called kinetic friction or dynamic friction.
 

Graph shows the variation of friction with applied force.

Rolling Friction

A body like a ring or a sphere rolling without slipping over a horizontal plane will suffer no friction, in principle. At every instant, there is just one point of contact between the body and the plane and this point has no motion relative to the plane. In this ideal situation, Kinetic or static friction is zero and the body should continue to roll with constant velocity.
 

In practice, this will not happen and some resistance to motion does occur. To keep the body rolling, some applied force is needed. For the same weight, the rolling friction is much smaller than static or sliding friction.
 

During rolling, the surfaces in contact get momentarily deformed a little, and this results in a finite area of the body being in contact with the surface. The net effect is that the component of the contact force parallel to the surface opposes motion.

How to reduce the friction?

Lubricants are a way of reducing kinetic friction in a machine.
 

Using ball bearings between two moving parts of a machine is another way to reduce the friction; since the rolling friction between ball bearings and the surfaces in contact is very small, power dissipation is reduced.


A thin cushion of air maintained between solid surfaces in relative motion is another effective way of reducing friction.
###TOPIC##Laws of Friction###

Laws of Static Friction

  1. Limiting friction depends on the nature of the surface in contact and their state of polish.
  2. It acts tangential to the two surfaces in contact and is opposite to the direction of motion of the body.
  3. The magnitude of limiting friction is independent of the area of contact.
  4. The limiting friction fms is directly proportional to the normal reaction N. i.e., fms N, fms = μsN.μs is called coefficient of static friction.
From the graph shown above it can be seen that, Fs < fms .i.e., Fs < is called coefficient of static friction.

From the graph shown above it can be seen that, Fs < fms .i.e., Fs < μs N. The general relation is Fs μk N, where μk is the constant of proportionality and is called the coefficient of kinetic friction.

Laws of Kinetic Friction

  1. The force of kinetic friction fk is independent of the area of contact so long as the normal reaction remains the same.
  2. It is independent of the velocity of sliding provided the velocity is neither too small nor too large.
  3. It is proportional to normal reaction N i.e., fk N, fk = μk N, where μk is the constant of proportionality and is called the coefficient of kinetic friction
    Fk < fms so μk < μs




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