Laws of friction

What Is Friction?

You want to move a box across the floor of your room, so you give it a strong push and then let go. You expect it to keep going all the way to the other side of the room because you learned in physics class that once an object is in motion it will continue to move in a straight line at constant speed until it's acted on by another force (in other words, Newton's First Law of Motion).

However, that's not what happened! Once you let go of it, the box slowed down and stopped. Does this mean that Newton's First Law isn't true? No, it just means that there is some other unseen force acting on the box: a friction force, slowing it down and eventually causing it to stop. There are other kinds of friction as well, like fluid friction and rolling friction, but in this lesson, we will mainly look at the friction force between two solid surfaces that are trying to slide past each other, a force called sliding friction.

There are five laws that help you understand and calculate sliding friction forces. We'll take a look at them now.

The First Law of Friction

The first law of friction says that the amount of friction is proportional to the normal force exerted between the surfaces. What exactly is a normal force?

The normal force (N) is a support force that is exerted on the object by the surface it's sitting on. It's the force that keeps you from falling through your chair when you sit down. The normal force can change based on the weight of the object and the angle of the surface it's on (if the surface isn't flat).

As the normal force increases, the friction force also increases proportionally. This is why it's harder to push heavy objects than light objects.

The Second Law of Friction

Although the friction force depends on the normal force, the second law of friction says that friction force does NOT depend on the area of contact between the object and the surface. Imagine if you turned a box so that less of it was touching the floor. Would that change the normal force? No! The box still weighs the same, so the normal force doesn't change, even if the area of contact does.

The Third Law of Friction

The third law of friction says that, in addition to the normal force, the friction force also depends on the type of surfaces in contact. For example, you know that if you try to push a heavy box across a carpeted floor, it will be harder to push than if you push it across a smooth floor. This is because the friction force between the box and the rough carpet is a lot bigger than the friction force between the box and the smooth floor.

To quantitatively measure the effect of different surfaces on the friction force, you can calculate the coefficient of friction. The coefficient of friction is a number that does not have any units that depends on the roughness of the two surfaces in contact. The symbol for the coefficient of friction is the Greek letter mu, and it typically ranges from 0-1. Smooth surfaces will have a coefficient of friction closer to zero, and rough surfaces will have a coefficient of friction closer to one.