Question

Grease and oil applied between moving parts of a machine increase the friction between them.

Why do athletes wear shoes with spikes, and why do tyres of vehicles have grooves (treads)?

In an activity, a wooden block is pulled with a spring balance across three surfaces. The reading just as the block starts to move is: glass sheet 2 N, wooden table 3.5 N, sandpaper 6 N. (a) On which surface is friction greatest? (b) Arrange the surfaces in increasing order of friction. (c) What do you conclude about friction?

'Friction is both a friend and a foe.' Justify the statement with two examples of each, and suggest two ways of reducing friction where it is not wanted.