Question 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?

Question 2: A person of weight 100 kg standing on skis 2 m long and 0.10 in 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)

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

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