Friction between two blocks

1. Oct 1, 2011

burhan619

I have a question for two scenarios (see the diagram attached):

1) When the two blocks on the table are at rest, the static friction the table exerts on the bottom block is counteracting the tension in the rope, resulting in a net force of zero. Is there any static force being exerted by the bottom block on the top block? I don't think there should be any, since there is no horizontal force acting on the top block.

2) Suppose M was massive enough to start descending, overcoming the static friction threshold of the bottom block. Also suppose that the top block starts to slip away from the bottom block. Which way does the kinetic force the bottom block exerts on the top block point? And what's exerting the horizontal force it's counteracting?

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2. Oct 1, 2011

Staff: Mentor

You are correct.

Friction acts in a direction to oppose slipping. Which way would that be?
What do you mean? This is kinetic friction.

3. Oct 1, 2011

burhan619

Is the free body diagram I've attached right?

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4. Oct 1, 2011

burhan619

here's a re-upload if that one's hard to see

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5. Oct 1, 2011

Staff: Mentor

I guess the color coding is to tell which body the force acts on.

The forces on m1 look OK.

On m2, you forgot the kinetic friction from m1 and the normal force from m1. Otherwise, looks good.

6. Oct 1, 2011

burhan619

That makes sense. Thanks.