Friction should be pretty straight forward. When non-smooth objects slide across a non-smooth surface, a force is created in the direction opposite to the direction of travel. But what I didn’t understand at first was where friction comes from for a rolling object. For a sliding object, the key word is sliding. Microscopic deformities on the surfaces of the objects will catch on each other. So the it’s the composite forces of overcoming all these deformities alongside other small forces like molecular attractive forces and even minor forces due to overcoming gravity for microscopic inclines.

But for a rolling object, there is no sliding. Theoretically, each point on the round object makes 1 to 1 contact with one point on the ground. So if we were to ignore friction from air resistance, what actually causes rolling objects to experience friction.

When I was younger, I was perplexed by how cars decelerated pretty quickly once you took your foot off the gas. I figured it was from the friction of the wheels on the axels. Because that is just pure sliding friction about a cylinder. But cars don’t just have wheels direct on the axels. They sit on bearings. And bearings work by having rolling pins or balls that spin between the inside and outside rings. So once again, there is no sliding.

I then learned that rolling friction, or rolling resistance, was a thing. So rolling friction exists because rolling objects are not perfectly rigid. Even steel bearings experience deformities as the balls roll around inside. The rolling object experiences compression and decompression as the points of contact touch the ground. This generates heat and thus energy loss, robbing the rolling object of its momentum. This naturally means that more rigid objects have a lower rolling friction than soft objects. This pretty easily demonstrable when you compare rolling a well pumped basketball and a slightly deflated one.

In terms of the actual forces resulting in decelerating the rolling object, it can best be understood by looking at this diagram of pressure distribution.

For this object rolling to the right, you can see asymmetrical pressure distribution. There exists greater forces opposing the direction the object is rolling in as it exists to the right side of the center of the rolling object along the x axis. Were the object to behave in a perfectly elastic way, the pressure distribution would symmetrical and thus not experience rolling friction.