What Causes a Sphere to Rotate on a Ramp with Negligible Friction?

[Kiwiro0ls]

If I have a sphere at rest on top of a ramp, if friccion is negligible, the only forces acting on it would be gravity and the normal force. Both of these go through it's axis of rotation which would be the spheres center. What would cause the rotation then so τ≠0?

 

[Doc Al]

What would cause the rotation then so τ≠0?

What makes you think it would rotate?

 

[Kiwiro0ls]

I guess there's no rotation at all unless there's friccion? boo...

 

[Doc Al]

Right. Without friction, the sphere will just slide.