Two blocks on a surface and a pulley

[Eitan Levy]

Homework Statement

Everything that can be used is in the picture.
μ is the coefficient of friction between both B and A and A and the surface.
A is moving upwards the surface with a certain acceleration.
B doesn't move in relative to the surface.
1. Why the direction of A's motion must be upwards and not downwards for B to not move in relative to the surface.
2. tanα=?

Homework Equations

ma=F

The Attempt at a Solution

I really don't know why A has to move upwards for this to be possible. If it move upwards both B and the surafce will apply force to the same dircetion (downwards the surface), but if it moves downwards they will both apply force upwards the surface. Why does it matter?
I thought that if I solved 2 maybe I would understand, however I can't seem to solve it.
I tried to draw a free body diagram, because if we want A to not move in relative to B we need them to have the same acceleration (in this case at least, correct?)
However, I have no idea how to draw the axes.
Any help would be appreciated.

 

[TSny]

if we want A to not move in relative to B

As I understand it, A does move relative to B. But B does not move relative to the incline.

 

[Merlin3189]

Think about all the forces on B in either case.

 

[Eitan Levy]

As I understand it, A does move relative to B. But B does not move relative to the incline.

I just can't type...

 

[Eitan Levy]

Think about all the forces on B in either case.

Friction, gravity and normal (the friction swaps its direction),
Still don't understand why it's impossible.

Maybe it's because at the second case B will accelerate downwards the sufrace for sure? And the surface can't accelerate the same way, so B will have to move in relative to it?

 

[haruspex]

Maybe it's because at the second case B will accelerate downwards the sufrace for sure?

Yes. If B were to stay put while A moves downplane underneath it then both friction and gravity would be acting down the slope on B, with nothing to balance them.