[" 7.When the surfaces in contact are made too smooth by "],[" polishing,force of friction "]

Choose the correct statements – (1) Force of friction is partically independent of microscopic area of surface in contact and relative velocity between them. (if it is not high) (2) Normally with increase in smoothness friction decreases. But if the surface area are made too smooth by polishing and cleaning the bonding force of adhesion will increase and so the friction will increase resulting in 'Cold welding' (3) Friction is a non conservative force, i.e. work done against friction is path dependent. (4) Force of fricton depends on area

Frictional force increases when surfaces in contact are made very very smooth. This is because

Why is the surface of a slide polished to make it smooth ?

(A) : A sphere cannot roll on a smooth inclined surface. (R ) : For a smooth inclined surface force of friction is equal to zero.

Now when the surface is smooth , the toy car travels larger distance. It is found that friction is caused by interlocking of irregularities on the two surfaces in contact . Irregularities between two surfaces in contact. The above figure shows minute irregularities on two surfaces which are locked into one another . If we want to move any surface , some part of the external force applied is used to overcome this interlocking want to maove any surface , some part of the external force applied is used to overcome this interlocking . A rough surface has a large number of irregularities , so the force of friction is greater . A smooth surface has small number of irregularities , so the force of friction is less. 2. Normal reaction : The force of friction depends on the normal reaction also. As the normal reaction increases , the interlocking between two surfaces in contact increases as they press harder against each other and hence friction increases .

When a body is moving on a surface, the force of friction is called