# Why is rolling friction less than the maximum static friction?

I know that rolling friction is almost always less than the maximum static friction, but why is this? In particular I am researching why ball bearings have much less friction. It has been said that the ball bearings come in contact with much less of the metal, but this does not make sense to me as friction is not proportional to surface area. So if it has nothing to do with surface area, then why is rolling friction generally much less than sliding friction?

• Im no expert, but I thought static friction was "rolling friction". I never heard of rolling friction as a term. Static friction is what keeps the contact point from sliding; sliding would fall under kinetic friction. This is the only time "rolling" as opposed to sliding is possible. Commented May 12, 2017 at 19:46
• The surface area must be involved. A perfect spherical surface would only touch a flat surface at 1 singular point at each angle of rotation, whereas a cube surface would have an entire area touching the flat surface upon sliding. Electrical forces and bumps in non perfect surfaces are involved here. I also belive a rolling motion of a perfect sphere only has surface friction similar to a single point touching a surface and then being interchanged with another point a small distance dx, further "down the line", therefor the static friction. Commented May 12, 2017 at 21:17

## 2 Answers

You are confusing "rolling resistance" and friction. These are two different unrelated phenomena, so there is no reason why one should be less than the other.

When the ball bearing rolls with constant velocity without sliding, there is no static or kinetic friction with the surface. The ball is not accelerating so there is no net force on it due to friction.

The ball bearing is slowed down because of rolling resistance, not friction. Rolling resistance arises from the deformation of the surfaces which are in contact, so unlike friction it does depend on the area of contact.

Rolling friction is nothing but energy lost by the object at the time of rolling , for a bearing (metal) it is primarily due to the static friction between two metals, but there are other properties like hysteresis which is caused by deformation of object , which is very negligible in case of bearing which is made of steel.

Now going little microscopic, the surface of ball bearings is uneven and rough and so the case of bearing is , so this roughness lock up together, more the surface area more they rough patch more they lock up so more the rolling resistance coefficient. so rolling resistance formula by itself doesnt look like they are dependent on surface area , but the rolling resistance coefficient is dependent on surface area which make rolling resistance dependent on surface area.