16.19 Why do things stop moving if we don’t push or pull them?
According to Newton’s second law (post 16.2), an object will keep on moving for ever unless we do something to stop it. To make a moving object slow down and stop, we have to apply a force (post 16.13) which is exactly what we do when we apply the brakes to stop a car. But a car will slow down, and eventually stop, if it runs out of fuel – even if we don’t apply its brakes. Is Newton’s second law wrong?
Let’s try something. Push an object along the top of a table – when you stop pushing, it stops moving. This happens whenever you push something along a flat surface. Everyday experience tells us that, the rougher the surface, the harder it is to push the object. The mass of the object, which provides its resistance to changes in motion (post 16.13), depends solely on the nature of the object itself, so cannot change when the object is placed on a rough surface. So something else must be stopping the moving object – because it changes the motion of an object, this “something else” is, by definition, a force.
We call this force, exerted by a rough surface on an object, friction.
Since objects are always rubbing against surfaces, in our everyday experience, we do not observe a world without friction. The nearest example is something moving along smooth ice. The frictional force is then very low and the object will continue to move for some time after you stop pushing it.
It is because friction is so common in our world that I have described ideas about motion for objects moving in space (post 16.4) – where they are not always in contact with surfaces and so do not always experience friction.
Why does friction exist? No surface is completely smooth. This includes the surfaces of all objects. Even a polished metal surface is not completely smooth when viewed in a scanning electron microscope – when it looks rather like the surface of the gravel in the picture above.
When we push the upper object, in the picture above, over the lower object, the lumps on one will get stuck in the hollows of the other. So lumps on one surface will push against lumps on the other surface. According to Newton’s third law (post 16.2), the lumps on the surface will push back, to oppose the motion of the object, and so exert a force on it. This force is friction.