Friction Direction in lever with drum

[Lisciu]

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.

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?

 

[davidmoore63@y]

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

 

[Lisciu]

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?

 

[Chandra Prayaga]

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