# Chapter 12 Class 8 <br>Science Question- answers

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## Friction

Introduction:

The study of friction is an important and complicated field of engineering and physics. When we push a box lying on the floor with a small amount of force, it does not move at all. It means that the surface of the floor, on which the box is resting, exerts some forces on the box which acts in a direction opposite to the force of our push. This natural force between the floor and the bottom of the box which opposes the motion of box on the floor is friction. The force acting on the two surfaces in contact which opposes the motion of one body over the other is called the force of friction.

Causes of Friction

The causes of friction are molecular adhesion, surface roughness and the ploughing effect. Every object has a rough surface, though the surface may appear to be smooth to the naked eye. But when we observe through a microscope, we find that the surfaces of all the objects have rough edges. Some of the particles on the surface of objects are in the form of tiny hills and valleys. When a body is pulled over another, these tiny hills and valleys present on both the surfaces in contact get entangled with one another.

(i) The nature of the two surfaces (smoothness or roughness of the two surfaces).

(ii) The weight of the body.

The frictional force depends upon: If the surface of an object is rough, more friction is produced and if the surface of an object is smooth, less friction is produced. Also, if the weight of an object is more, then friction is more and if the weight is less, friction is also less.

Types of Friction:

The different types of friction are:- (i) Static Friction (ii) Rolling Friction (iii) Sliding Friction

1. Static Friction

The opposing force that comes into play when one body tends to move over the surface of another, but the actual motion has not started is called static friction. Static friction comes into play when a body is forced to move along a surface but the movement does not start. The magnitude of static friction remains equal to the applied external force and the direction is always opposite to the direction of motion. We can demonstrate the static friction between a wooden block and a tabletop by performing an activity as follows.