Forces of rolling-Friction

[ctpengage]

P is the contact point of a wheel with the ground. An angular acceleration causes the wheel cause at P. There a friction forces must act on the wheel at P to oppose that tendency.
However when a wheel rolls down a ramp, there is a force F_gsin(a). This causes the wheel to accelerate. However it is plainly obvious that there is a friction force against the direction the wheel is rolling in.

Can anyone help me resolve this apparent contradiction. Thanks

 

[Mattowander]

Sure! All this is basically saying is that Fnet = Fgsin(a) - Frictional force. That is the only way the wheel will be able to accelerate downwards.

Frictional force here is most likely (coefficient of friction times mgcos(a)), where mgcos(a) is the component of gravity equal and opposite to the normal force.

I hope that helps.