One cause of friction is that nothing is actually smooth. If you were to look at just about any surface under a strong enough microscope, you would see lots of little bumps and ridges and things. When two things rub against each other, the little bumps catch on each other and slow the two objects down. The smaller the bumps and ridges are, the easier they will slide against each other, and the less friction you will get. If this doesn't make sense to you, think of rubbing two ice cubes together vs. rubbing two sheets of sandpaper together. The sandpaper is bumpier, so it has more friction.
Friction depends on a lot of different things, most of which Iím not going to try to get into right now. But one thing I can tell you is that there are actually two different kinds of friction between solid things touching each other, called 'static' and 'kinetic' friction. Static friction is the kind of friction you get if two things are already stationary or moving together. Kinetic friction is the kind of friction you have between two things that are moving against each other.
Static friction between two objects is usually larger than kinetic friction between the same two objects because two surfaces that stay together can really get stuck to each other, for example by bumps in one surface getting lodged in grooves in the other. That's one reason why it's often so hard to get something like a screw or a jar lid unstuck, even though it's not hard to turn once it's loose.
I found a really cool demonstration of this on Fisher Scientific's Teaching Tips Page
. You'll need a motorized toy car of some sort. Take some blocks or a board and set them at an angle so that the car just barely keeps from sliding down it. As long as the car is just sitting on it, there is static friction between the car's wheels and the board preventing it from starting to move. Then, turn the motor on (the car should be pointing /up/ the board). Now you will have kinetic friction between the car's moving wheels and the board. Since kinetic friction isn't as strong as static friction, the car will slide right down the ramp even though its wheels are pointing the other way!
(republished on 07/12/06)