4 - Friction

Summary

Together with gravity, friction forces are the ones which play the biggest role in shaping everyday life. Without friction, we would be unable to drive our cars, to walk, or even to hold our pens. The simple laws of friction on which our model is based seem to have been first stated by Leonardo da Vinci (1452–1519), who wrote prolifically about lots of things.

Although these laws for friction are very simple, they provide useful estimates and qualitative predictions for a wide range of behaviour associated with friction. More sophisticated models are sometimes used, however, in specialized areas (such as the design of bearings in engineering).

Coefficients of friction

Friction forces arise as a result of contact between two surfaces. A good way to get a feeling for these forces is to experiment with the two surfaces consisting of the top of the table and the palm of your hand.

An instructive experiment is to rest your hand on the table and then exert a gentle forwards pressure, but not enough to move your hand. You should then be able to feel the backwards reaction from the table opposing your forwards push. Now gradually increase the forwards pressure. The backwards reaction force builds up to a maximum. After this your hand slides forwards. The backwards reaction force which opposes your push in the forwards direction arises from the friction between the palm of your hand and the table.