Q.9 Give examples to show that friction is both a friend and a foe.

Advantages of friction:

(i) Walking on the ground is possible due to friction. As we step forward using one of our feet, then the ground surface and the foot surface gets interlocked between each other's minute roughness, as a result of which that particular foot gets a support to let the other foot come forward, thus making the walking process more easier.

(ii) Friction between the tip of the pen and a paper allows us to write as the tip surface of the pen and the surface of paper both have minute roughness by which they tend to get interlocked and thus the pen tip doesn't slip easily on the paper surface and this makes the writing process more easier.

Disadvantages of friction:

(i) Tyres and soles of shoes undergoes wear and tear because of friction as the surface of tyres and shoes have to overcome the friction provided by the ground while walking by preventing the roughness on the surface of tyres and shoes to get interlocked with the roughness on the surface of ground.

(ii) Friction between different parts of machines produces heat. In machines, there is sometimes rubbing of two surfaces of two different parts, due to which there is a rapid process of overcoming each other's roughness on the surfaces and this produces heat. Continuous heating at a particular surface may melt or result in wear and tear of those parts.