Cause of Friction
When we attempt to move one object over the other, we have to apply a force to overcome interlocking of the irregularities in their surfaces. More the roughness of a surface, larger is the number of irregularities on its surface and hence greater will be the friction. Thus, the force of friction is greater if very rough surfaces are involved.
In Figure 9 we have shown a highly magnified diagram of the surfaces of two wooden blocks A and B kept one over the other. Please note the rough surfaces of these wooden blocks having lot of irregularities. When we pull the upper wooden block A over the lower wooden block B by applying a force, then the irregularities of their rough surfaces get entangled (or locked) with one another. This interlocking of surfaces gives rise to force of friction which opposes the motion of upper block A over the lower block Bl We are able to move block A over block B because we apply sufficient muscular force while pulling to to undo the interlocking of surfaces and overcome the opposing frictional force. The block A moves over block B only when the pulling force applied by us becomes greater than the force of friction holding them together. Friction is small for smooth surfaces (like glass and ice). Friction is much greater for rough surface (like sand paper and concrete).
Before we go further, we should know what a spring balance is because it will be used to perform some activities based on friction. The spring balance is a device which is used for measuring force acting on an object. The spring free end (having a hook). The stretching of spring gets stretched is a measure of the force applied. Larger the stretching of spring, greater will be the magnitude force applied. The stretching of spring or magnitude of force is indicated by a pointer attached to the spring which moves on a graduated scale. The reading on the scale of spring balance (as indicated by the position of pointer) gives us the magnitude of force. When the spring balance is held vertically (as shown in Figure 3), it is said to measure the weight of an object hung from its hook (because weight of an object is also a force). And when a spring balance is held horizontally (attached to an object and pulled), it can be used to measure the force being applied to pull the object on a horizontal surface.
FACTORS AFFECTING FRICTION
It has been found by experiments that the friction between two surfaces depends on two factors :
1. The nature of the two surfaces (smoothness or roughness of the two surfaces).
2. The force with which two surfaces are pressed together.
The force of friction, however, does not depends on the ‘amount of surface area’ of the two objects which is in contact with each other. We will now study how the friction depends on the nature of two surfaces as well as on the force with which the two surfaces are pressed together.