PFE008: Friction of Tires

The friction force is something that we all experience

(or don't). In spite of the commonplaceness of the friction force, it isn't listed as one of the four fundamental forces: gravity, electromagnetic, strong nuclear, and weak nuclear. Actually none of those even sound like friction. Where's the "makes-the-box-hard-to-push-across-the-floor" force? As it turns out, friction is actually the electromagnetic force (think electrical wires, magnets, shocks, light, microwaves, etc) except on a person sized scale.

The physics of what's actually happened can be reasonably thought of as the actual atoms of the box pushing against the atoms of the floor. More precisely, the electron clouds

of each atom push against each other. Since they both have the same charge (negative) they repel. This is a perfect example of Coulomb repulsion which comes out of the electromagnetic force.

But how does this apply to big boxes full of books? Let's think about what we know. If you fill up the same box on the same floor with more stuff, it's going to be harder to push. So clearly the friction force is proportional to the weight of the object. We also know that it is going to move more easily on hardwood floors than on carpet. So the materials sliding past each other is important too. This actually completely describes the equation. The force of friction is the weight of the object times a constant that describes the materials (typically of values around one half) known as the coefficient of friction. Nothing more complicated than that.

Actually, I lied. But only a little. The constant depends not only on the two materials, but also whether they are sliding past each other (dynamic) or not moving (static). But you knew this already too! When you start pushing on the box, you push harder and harder, until it suddenly starts to move, and then it's moving a whole lot! That is, the static coefficient of friction is (almost always) greater than the dynamic coefficient of friction. In other words, it's easier to keep the box moving once it's started moving, than to get it to move in the first place.

So now you know basically everything there is to know about friction, from a theoretical point of view. Let's apply this to some real life examples.

First: cars. The friction between tires and the road is consistently a source of confusion. For the moment, we will assume that your tires aren't slipping (like when you spin out, or on ice). In this case, your tires experience static friction with the ground.

This may sound misleading because, hey! My car is going fast! There's nothing static about my tires! But remember, static friction is only concerned with the relative motion of the two surfaces. At the point of contact with the ground, your tire is not moving relative to the ground. If it were, your wheels would be spinning and your car not going anywhere (i.e. spinning out). This static friction is what makes your car go forward! Your wheels push against the ground, and the ground pushes back, making your car go forward (and, actually, making the earth spin a very tiny bit in the opposite direction. Don't worry, I did the math (pdf) and we're all good.).

It's interesting to note that the limiting factor in drag racing,

is the amount of friction they can get between the tires and the road.

The next interesting friction example, is ice. Ice is super slippery. I mean, I can't think of another solid that slippery at all. So what makes ice so special? It actually has to do with what makes water so special.

The above discussion of friction only applies when both contact layers are solids. As it turns out, when you're slipping on ice, whether in shoes, on skates, or in your car, you are floating on a very thin layer of water.

This thin layer of water comes out of a bizarre property of water that is true for very few materials. Generally, when something freezes (goes from a liquid to a solid) it gets smaller. The atoms get closer together and they move less. Water does the opposite. When it's in ice form, it forms a structure (which is the same reason why snowflakes are always so pretty) so it actually expands while freezing, or shrinks when it melts. (This is why pipes freezing=bad.) That means that the liquid form of it is actually a bit smaller, so when you step on it, your weight will actually cause a teeny bit of ice on the surface under where you're standing, to turn into water. This is what allows you to slip, slide, or spin out of control.

That's friction.