# Block on Block with Friction

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1. May 16, 2015

### minimario

1. The problem statement, all variables and given/known data

2. Relevant equations

F = ma

3. The attempt at a solution

When drawing a FBD for block 1, I couldn't figure out if the friction force between the blocks was toward the left or toward the right. Similar situation on block 2. How would I determine this?

2. May 16, 2015

### Staff: Mentor

Which direction do you expect for the acceleration of the top block?

Technically you can choose both directions for the force, but one choice will give a negative value for the force.

3. May 16, 2015

### minimario

Accelerate left. So the friction force would be left?

4. May 16, 2015

### TSny

Which way do the dishes accelerate in this video?

5. May 16, 2015

### minimario

To the right, but when I put a book on top of a heavier book and pull the heavier book right, the book goes left and falls off?

6. May 16, 2015

### TSny

No, the top book goes right. It falls off the left end of the bottom book because the bottom book accelerates faster than the top book.

7. May 16, 2015

### minimario

Oh. So the friction on the top book would be right, and that on the bottom book would be left.

8. May 16, 2015

### TSny

Yes, that's correct.