Friction force for a rolling wheel?

AI Thread Summary
Friction acts on the bottom point of a rolling wheel, opposing any velocity, including both linear and rotational motion. When a wheel moves down a ramp, the friction force acts up the ramp to prevent slipping, despite the rotational motion also being up the ramp. The concept of "sliding" refers to the absence of rotational motion, where friction behaves differently. Understanding the direction of friction requires analyzing the velocity of the bottom point of the wheel. This clarification helps in grasping the dynamics of rolling motion and friction's role in it.
quincyboy7
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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.
 
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quincyboy7 said:
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.
 
Go to the link in "direction of friction" in the above post to find out more.
 

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