Class VIII
Friction
Content

• Force of friction
• Static friction, sliding friction and rolling friction
• Fluid friction

A force, which is responsible for changing the state of motion of objects is known as

force of friction.

It is always opposes the applied.

Friction is arises due to the irregularities on the two surfaces in contact. Those surface which appear very smooth have a large number of minute irregularities on them, on rough surfaces. There are a large number of irregularities

Thus the force of friction is greater if a rough surface is involved.

Fig (1)
(irregularities surface)

The force of friction increases if two surfaces are pressed harder.

The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.

The force required to keep the object moving with same speed is measure of

sliding friction.

The sliding friction is slightly smaller than the static friction.

Without friction no object can stop, if is moving.

Friction can produce heat e.g.

Stricking a match stick produce fire by friction, Rubbing of your palms makes you feel warm.

The substance which reduce friction are called

lubricants.

It is used in machine as oil to reduce friction. An air cushion between the moving is also used to reduce friction.

If one body rolls over the surface of another body, the resistance to its motion is called

rolling friction.

It is always easier to roll than slide body over another.

The rolling friction is smaller than the sliding friction. So sliding is replaced in most machine by rolling by the use of

ball bearings.

Examples

are the use of ball bearing between hubs and the axles of the ceiling fans and bicycles

as shown in figure. It is reduce friction.

Fig (2)
(ball bearing)

The common name of the gases and liquids is

fluid.

It can also exert of friction when object move through them. This is known as

fluid friction.

The frictional force exerted by fluid's is also called

drugs.

The frictional force on an object in a fluid depends upon its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid