[Physics] Direction of Friction on two stacked boxes

free-body-diagramfrictionhomework-and-exercises

I have box 2 stacked on box 1. Box 1 is on a frictionless table and accelerated by a force. However, there is friction between box 1 and box 2.

Now, I was told that this is the diagram for that problem:

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I thought that since m1 and m2 are moving to the right, then the frictional force between box 1 and 2 will be to the left, opposite to the direction of motion. Can someone confirm what is the actual direction of friction for the frictional force between box 1 and box 2?

Also, is the action-reaction pair of forces between box 1 and box 2 the normal force of box 1 and the negative normal force of box 1?

It is the same issue with another problem I saw online:

enter image description here

In one of the free-body-diagrams, the friction seems to be going to the right (which makes sense since the movement is to the left), but in the lower free-body diagram the friction appears to be going to the right (in the same direction of movement). How is this possible?

Best Answer

Consider Newton's Third Law; commonly stated "every action has an equal and opposite reaction".

You should expect the surfaces to have friction in the opposite direction. Friction doesn't help one of them move, it resists the relative movement of both, causing forces in opposite directions.

You can sense this yourself by taking two rough surfaces and trying to move them one with each hand. The friction will resist movement with both hands, not just one.

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