Tie a string around a brick. Pull the brick by a spring balance Fig. You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the force of friction between the surface of the brick and the floor.
Now wrap a piece of polythene around the brick and repeat the activity. Do you observe any difference in the readings of the spring balance in the above two cases? What might be the reason for this difference? Repeat this activity by wrapping a piece of jute bag around the brick. What do you observe?
It is observed that the reading of the spring balance in the case when brick is wrapped with a piece of polythene is less than the other cases.
The reason for this difference could be that in the first case, since the brick surface in contact with the ground is rough, the surface gets greater force of friction. But in second case, when the brick is wrapped with polythene, the surface gets smoother. Hence, less frictional force is required to move it.
By wrapping a piece of jute bag around the brick, it is observed that the reading of spring balance is more than the second case, when the brick was wrapped with polythene.