1) It is observed that snow lies stably on roofs with a slope of less than 24°. but that It slides off roofs with a greater slope. Skiers, on the other hand. slide on a snow-covered mountainside with a slope of only 2°. Why is this?

2) A person of weight 100 kg standing on skis 2 m long and 0.10 m wide slides on the 2° mountain slope. at 0°C. Calculate the loss of potential energy when the ski slides a distance equal to its own length. Hence calculate the average thickness of the water film beneath each ski. (The latent heat of fusion of ice is 330 MJ m'3.)

3) Give examples, from your own experience, of situations where friction is (a) desirable, and (b) undesirable.

4) Give examples, from your own experience, of situations where wear is (a) desirable. and (b) undesirable