It’s become my understanding that a lot of people don’t really understand friction. That’s Ok, I hazard to guess that no one really understands friction when it’s first explained to them. If you think you understood friction when it was first explained to you, you were probably missing something.
Let’s look at how we model friction. We view it as a force which tends to oppose relative movement between two objects in contact with each other. Say your mother asks you to pass the salt, so you (being a somewhat lazy daughter) push the shaker to her. The salt is sitting on the table, so they are in contact. You only want to move the salt, not the table, so you’re trying to move the shaker relative to the table. You push it very gently, but nothing happens. You are applying force to the shaker, but friction is applying force to counter this force. Specifically, we call this the static friction force (static because the shaker is stationary). The equation all high school physics teachers (and teacher’s pets) can quote is:
- Fs equals the force due to static friction. It’s actually the maximum force that static friction will apply; the actual force will be just large enough to counter the force you’re pushing with, no more.
- Fn equals the normal force. Normal, in this case, means, "90° to the surface" not "2.5 kids just like everyone else". Gravity is pushing down upon the salt, yet the salt does not fall because the surface of the table is pushing back up. This is the called the normal force.
- μs equals the coefficient of static friction (again, static because the shaker is stationary). This depends on the nature of surfaces in contact. Slippery surfaces have a low coefficient, sticky or rough surfaces have a high one.
So the shaker isn’t going to move until you push hard enough to overcome the static friction force, which you pretty much calculate by taking the weight of the shaker (because the weight happens to equal the normal force) and multiplying it by some number (the coefficient of static friction). Or you could just increase the pressure you’re pushing with until the shaker starts to move.
Now (since you’re lazy), you’ll be glad to know that once you’ve started it moving, you don’t have keep pushing quite as hard. That’s because objects in motion generate kinetic friction rather then static friction, and kinetic friction (also known as dynamic friction is weaker. You use a similar equation to calculate kinetic friction.
The coefficient of kinetic friction is less then the coefficient of static friction. Because you’re multiplying by a smaller number (and the normal force is the same), the resultant force is smaller. As long as you keep pushing harder then the kinetic friction force, the salt will keep moving. So you might have to push hard to start a heavy object moving, but you won't have to push as hard as long as it's in motion. Of course, if it stops you'll again have to push hard to start it again.
Thusfar, you might not like friction, since you’ve been wanting to move the salt and it’s been stopping you, but once the salt reaches the end of its journey (in front of your mother), you’ll be grateful friction is there. Without friction the salt would keep on going even once you’d stopped pushing on it (remember Newton’s laws of motion?) and spill into your mother’s lap; she’d hate that.
So we have some equations to model friction which work in many situations (though not all, see below). Do they make sense? One weird thing is that the only the weight of the object matters, not the size or how much of the object is in contact with the surface. This does in fact make sense if you understand the underlying cause of friction.
Some form of electrostatic bond (hydrogen, ionic, or Van der Waal most likely) will tend to form between most any two molecules which get sufficiently close to each other. These bonds hold solid objects together, but will also form between two objects. The trick is "sufficiently close" happens to be a very small distance. When one flat object rest on another (like a piece of paper on a desk) it looks like their surfaces are quite close to each other, but this is largely an illusion. On an atomic scale, few surfaces are very smooth. Bumps far smaller then we can see loom like mountains to an atom. A piece of paper has hundreds of juts and protrusions sticking out of its surface. When a piece of paper rests on a desk, only a few of these protrusions are actually in contact with the desk (or rather, the desk’s protrusions, since the desk to no flatter then the paper). Only the molecules at the end of these protrusions are close enough to form these electrostatic bonds. These molecules will cold weld. If the paper is to be moved across the surface of the desk, these cold welds, must be broken. The force of friction is really the force of these cold welds.
Imagine a stool with 100 legs, none of which are the same length. It will rest on only three of them (because three points define a plane). Now if you pressed on the top of this stool, it might twist it so that more legs come in contact with the ground. This is why the weight of the object plays such a big role in our model of friction. Heavier objects have more force pushing them against the surface, so more protrusions come in contact with the surface, more cold welds are formed, and more force must be applied to overcome them. Making such a stool wider will not (by itself) bring more legs into contact with the ground; so increasing the surface area will not cause more protrusions to connect, so will not cause an increase in the frictional force. The coefficient of friction is an expression of how "twistable" the surface of the object is. You have to press very hard to make more legs of a stiff stool touch the floor; surfaces with a low coefficient of friction are similarly "stiff" in this respect.
As I said, the model introduced above is a good one, but it does break down:
- The model really only applies to solid object, yet fluids cause friction too. We call friction caused by fluids "drag", and have to use a very different set of equations to model it.
- The model assumed that neither surface enters the other, yet a narrow object might dig into a soft surface. That’s why deep treads, chains, or spikes might give you better traction in snow or sand.
- The model assumes neither surface is especially compressible, yet snow (and to a lesser extent, sand) will become packed as you drive on it. Some force is spent compressing the material, so the frictional force is effectivally reduced. Wider tires spread the weight of a vehicle around, so less of it is compressing any given inch of snow, the snow gets less packed, and the tires get better traction.
- The model assumes relativally low speeds. At higher speeds, the velocity of the object must also be factored into the situation.