Friction Forces Between Stacked Blocks: Determining Direction

[minimario]

Homework Statement

Homework Equations

F = ma

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?

 

[mfb]

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.

 

[minimario]

Accelerate left. So the friction force would be left?

 

[TSny]

Which way do the dishes accelerate in this video?

 

[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?

 

[TSny]

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?

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.

 

[minimario]

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

 

[TSny]

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

Yes, that's correct.