On car tires, shoes and other such items, having less of the material in contact with the surface underneath increases traction. Why is that? Isn’t friction a function of the contacting area and speed?
In: 11
Actually no, friction is *not* a function of the contacting area and speed. It is a function of weight (i.e. the force pushing straight down onto the surface) and the coefficient of friction between the two materials (e.g. rubber on dry asphalt has a high coefficient of friction while wood on teflon has a very low coefficient of friction).
The coefficient of friction is always higher when the two materials are not sliding on each other. When they start sliding, the “static” (not moving) coefficient of friction changes to a much smaller “kinetic” (moving) coefficient of friction.
To feel this in practice, try the following:
Standing in sock feet on a slippery floor, try to push yourself away from a wall until you start sliding. Now lift one foot, and try again. Chances are you will need just about exactly the same amount of force to start sliding on two feet as you will when you’re standing on one foot.
When you are standing on one foot, there is only half the surface area (one foot vs. two feet) to keep you from sliding, but on the other hand, when you are standing on two feet there is only half of the weight per foot.
Latest Answers