## Rolling Motion w/Static Friction

Hey guys, I need help understanding why it is static friction that is involved in pure rolling motion?

From what I remember in translational motion, when the maximum static friction force is overcome, then the given object/particle/body will start accelerating. Otherwise, the object/particle/body will just stay put right?

Anyhow, how does static friction determine a rigid object under rolling motion to undergo a "pure rolling motion?" Wouldn't the static friction force slow down the object or doesn't it have to be overcome for the rigid object to start rotating about its center of mass?

I just cannot seem to picture it in my head right now and if you could help explain this concept better, then I can progress in my understanding of "pure rolling motion."

Thanks.
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 Mentor Most mechanisms of moving along the ground use static friction rather than dynamic friction. Generally if you are in a situation where dynamic friction is involved you are either about to fall on your rear-end or slide off the road. Whether you are walking or driving, the important point is that there is no sliding "where the rubber meets the road".
 Think of a toothed wheel rolling on a toothed rack. That's a rack and pinion set-up in engineering talk. Obviously the wheel won't slide, but if the teeth are small relative to the radius, the wheel will roll quite smoothly with only the gentlest push. Static friction is felt by the wheel as it starts rolling. The frictional force at the rim opposes the push at the central axle (you are doing the pushing) and this couple starts the wheel rotating. There is only dynamic friction in this example if the axle sticks and the push is enough to disengage the teeth and grate the wheel along without rotating.

## Rolling Motion w/Static Friction

For a rolling motion to start, you need a torque. This torque comes from static friction, which as you said tries to slow down the object.

So try to think what happens when you (or gravity) pulls at the center of the ball: it tries to move forward because there is a force. Which means that the friction will try to slow it down by applying a force at the bottom of the ball. Since this force acts at a distnace, you get a net torque. Net torque means the ball starts rolling.

Now imagine you let go of the ball, so there is no net force on the ball, so the ball keeps rolling at a constant rate. It does not slow down, because as DaleSpam pointed out above "there is no sliding where the rubber meets the road". In an object that is rolling at a constant rate, the velocity of the bottom relative to the ground is 0. Since friction only tries to overcome relative motion and there is none, then there is no friction.

Of course, if there was relative motion between the bottom and the ground (this is called rolling with slipping), then there will be dynamic friction and the angular velocity of the ball will decrease. Just try to spin a hoolahoop very fast before dropping it on the ground, and you will see.

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