Explain the Friction

Friction is a force that works against all motion. It slows everything down and it is the reason that the motion we are most familiar with always seems to slow down and stop. It occurs any time two surfaces move against each other.

On a cold morning you might rub the palms of your hands together and use the heat created by the friction of your hands' rubbing to make your hands feel warmer. The brakes in a car use the force of friction to bring your car to a stop.

In your experience, do some situations have more friction than others? What does it depend on? Let's compare two situations - sliding a heavy box of books on a tile floor or on a carpet. In which situation would you need less force to move the box? On the tile, of course. Why? Because a smooth surface such as tile offers less friction when sliding a box than a rough surface like carpet. Can you think of a surface that would offer even more friction to sliding? How about blacktop or asphalt? So the amount of friction depends on how rough or smooth the surface is.   

What else does the force of friction depend on? What if you are sliding an empty box or the same box loaded with books, which is easier? The empty box, of course. Less weight pushing down on the surface means you have to use less force to overcome friction. This leads to one way frictional forces can be represented as:

F_f = μN

where F_f is the frictional force in Newtons, μ is the coefficient of friction (pure number) and can range from .01 to .99 with larger values for rougher surfaces and N is the normal force in Newtons, the force that the surface holding the box pushes up against the box. The frictional force is always in the direction opposite to any motion.