Friction on a rolling body

Question

I have been reading about rolling motion and I seem to have a confusion regarding the friction acting on such bodies. First off for a body to start rolling their has to be some friction b/w the surfaces in contact. (right? Otherwise it would simply slip on the surface.) I also know that friction acts in a direction which opposes relative motion (correct me if this statement is incomplete.) Now suppose a body (say a sphere) is performing a pure roll, i.e it doesn't slip on the surface. At the point of contact between the sphere and the surface the rotational and linear velocities are equal and opposite in directions ($v=wR$) so the point of contact is essentially at rest and there is no relative velocity b/w the the point of contact and the surface,and thus NO FRICTION acts on the sphere. (pure roll.) So my question is how does the sphere slow down eventually? For it to slow down friction has to act opposite to the motion of the sphere but why does this friction act if there is no relative motion between surfaces in contact (i get that the entire sphere has a relative $v$ wrt to the surface but the friction acts at point of contact which is a rest.) Maybe I am thinking too ideally and in reality other factors are also at play, if so what is the more realistic system?

Answer 1

If there were no drag or losses, then you're correct. The sphere would move forward and rotate indefinitely.

But assuming the surfaces are not completely smooth and there is air in the room, then there will be drag forces on the sphere. These forces slow down the sphere relative to the surface.

On a frictionless surface, this slowdown would cause the rotation and the motion to diverge. The sphere would be spinning such that the bottom is moving backward relative to the sphere's forward motion.

But on a normal surface, frictional forces appear that push on the sphere. Among other effects, this acts to slow the rotation.