Friction force for a rolling wheel?

[quincyboy7]

Why does the friction force oppose only the rotational motion of the bottom of a wheel? Also, when a wheel is moving down a ramp, why is the friction force UP the ramp when the rotational motion of the bottom wheel is also up the ramp? What is "sliding"?

I'm having a real hard time understanding these and any help would be much appreciated.

 

[ashishsinghal]

Why does the friction force oppose only the rotational motion of the bottom of a wheel?

This statement is incorrect. In many cases friction imparts rotational motion. Motion of a wheel down th ramp is such an example.
Actually friction acts on the bottom most point of the rolling object. Hence it will oppose any velocity (not only angular velocity) which is the usual role of friction. So to get the direction of friction you need to first find the direction of velocity of the bottom most point.
Slide is the situation when no rotational motion takes place.

 

[ashishsinghal]

Go to the link in "direction of friction" in the above post to find out more.