Homework help regarding forces of friction and their directions

[MechRocket]

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! :)

 

[Saladsamurai]

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?

 

[m2003]

I can explain the direction of the forces of friction in this scenario. First, let's define what friction is. Friction is a force that occurs when two surfaces are in contact and moving against each other. It always acts in the opposite direction of the movement, and its strength depends on the type of surfaces and the force pressing them together.

In this problem, the block and the slab are in contact and moving against each other. Therefore, the force of friction acts in the opposite direction of the movement, which is to the left for both the block and the slab. However, the force of friction also depends on the force pressing the surfaces together. In this case, the weight of the block is pressing down on the slab, creating a normal force (N) that is perpendicular to the surface of the slab.

The force of friction is directly proportional to the normal force, meaning that the stronger the normal force, the stronger the friction force. In this case, the normal force on the slab is stronger than the normal force on the block because the slab is larger and has a larger surface area in contact with the ground. Therefore, the force of friction on the slab is also stronger than the force of friction on the block, and it is pointing in the same direction as the movement of the slab, which is to the right.

I hope this explanation clears up any confusion you had about the direction of the forces of friction in this scenario. Remember, the direction of friction always opposes the movement, but its strength is also affected by the normal force between the surfaces. Keep up the good work with your homework!