Friction Direction in lever with drum

AI Thread Summary
The discussion centers on understanding the direction of friction forces in a lever system with a rotating drum. It highlights that friction acts opposite to the direction of relative motion between surfaces in contact, which can lead to confusion when analyzing free body diagrams. In the case presented, while the wheel rotates counterclockwise, the lever moves to the right relative to the wheel, resulting in friction acting in the same direction as the lever's motion. This phenomenon is explained through Newton's Third Law, emphasizing that friction forces between interacting bodies are equal and opposite. Ultimately, the key takeaway is that the direction of friction depends on the relative motion of the surfaces involved.
Lisciu
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Hi guys,

I trying to understand one thing that I don't know where came from. To visualize the problem I will used the drawings from book.

9ZDO4vV.jpg


The question is for direction of Force Fb on drawing (b) and (a). I was always learned that friction is acting on direct opposite to motion. In drawing (b) we have correct direction because the wheel rotate counterclockwise so friction must act to right side to stop the motion. But we going to free body diagram in (a) and we see that friction is acting on the same direction as motion...Could someone me explain like for child why actually this happen?
 
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Here 2 ways to say it:
(1) Relative to the wheel, the Lever is moving to the right. So the force Fb is indeed opposite to the motion
(2) When two bodies interact, the friction forces they exert on each other must be equal and opposite by Newton's Third Law
 
So basically it was because of the Third Law of Newton? It have nothing to do with the point of frame ? And it was established on the wheel because it was easy way to find the proper direction?
 
Lisciu said:
So basically it was because of the Third Law of Newton? It have nothing to do with the point of frame ? And it was established on the wheel because it was easy way to find the proper direction?
What matters is the direction of relative motion between the surfaces in contact. Since you agree that the wheel is rotating left with respect to the lever, the lever surface is moving to the right with respect to the wheel
 
This has been discussed many times on PF, and will likely come up again, so the video might come handy. Previous threads: https://www.physicsforums.com/threads/is-a-treadmill-incline-just-a-marketing-gimmick.937725/ https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/ https://www.physicsforums.com/threads/how-do-we-calculate-the-energy-we-used-to-do-something.1052162/
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