# Friction between two blocks on a table

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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## Answers and Replies

Doc Al
Mentor
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.
You are correct.

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?
Friction acts in a direction to oppose slipping. Which way would that be?
And what's exerting the horizontal force it's counteracting?
What do you mean? This is kinetic friction.

Is the free body diagram I've attached right?

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

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Doc Al
Mentor
Is the free body diagram I've attached right?
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.

That makes sense. Thanks.