Homework help regarding forces of friction and their directions

AI Thread Summary
The discussion revolves around the forces of friction acting on a block and a slab. There is confusion regarding the direction of the friction force on the slab, with the assumption that it should point left, opposing the block's movement. However, the correct solution indicates that the friction force on the slab actually points to the right. This is clarified by considering the interaction between the two surfaces, where the slab's motion influences the friction direction. Understanding these dynamics is crucial for accurately analyzing forces in physics problems.
MechRocket
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The Problem

http://img15.imgspot.com/u/07/118/16/friction.JPG

What I Think

http://img15.imgspot.com/u/07/118/16/think.JPG

The Correct Solution

http://img15.imgspot.com/u/07/118/16/solution.JPG

What Is Confusing Me

Shouldn't the force of friction on the slab be pointed to the left? I knew the force of friction on the block is pointed to the left (because friction will oppose movement), so I naturally assumed this force to the left also acts on the slab (the two surfaces are touching after all).

I just cannot see why the friction force on the slab is to the right.

Thanks a lot! :)
 
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Picture instead of the slab remaining stationary and the block sliding right, the block standing still and i the slab is sliding left. . . then which direction will oppose motion direction?
 

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