Important Questions for Class 8 Chapter 12 - Friction

Important questions based on NCERT syllabus for Chapter 12 - Friction:

Question-1: Why friction even exists between two smooth surfaces? Why force of friction is greater if a rough surface is involved?

Solution: A. Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome interlocking.

B. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.

Question-2: Answer the following:
A. Why friction is called a necessary evil?
B. Explain why objects moving in fluids must have a special shape?

Solution: A. Friction is necessary in everyday life for us to start moving, playing and various activities. At the same time it also hinders motion and heats up machineries and creates wear and tear. So it’s necessary evil.

B. When objects move through fluids, they have to overcome the friction acting on them. Efforts are therefore made to minimize the friction so objects are given special shapes. Idea of such shapes come from the body structure of birds and fishes which have to move about in fluids all the time. Such shapes are called streamlined. Giving such shapes to the ships, plane and cars is called streamlining.

Question-3: Mention three methods by which we can reduce friction between two bodies.

Solution: Use any of the following methods to reduce friction:

(I) By applying a lubricant.
(II) By using ball bearing.
(III) By polishing surfaces.
(IV) In the machines where oil cannot be used air cushion between the parts is used to reduce friction.
(V) By streamlining.