What is the direction of the force, up or down?

[asi123]

Homework Statement

I have 2 mases, m1 and m2, goes up the surface with acceleration "a".
Between the mases there's static friction which cause the power f_s.
My question is, what is the direction of the force, up or down?
I always though that the power should be directed up, because m1 tries to slide back, so f_s has to hold it, but in the solution of this problem they wrote the opposite, so I'm pretty confuse.
10x.

Homework Equations

The Attempt at a Solution

 

[Doc Al]

What's accelerating the blocks up the incline? Is there an applied force? Where is it applied?

 

[asi123]

Yeah, sorry.
Here is a better one.

 

[Doc Al]

Much better diagram!

Between the mases there's static friction which cause the power f_s.
My question is, what is the direction of the force, up or down?

That depends on which block you are talking about. m1 and m2 exert equal and opposite friction forces on each other. One force will be up the incline, the other will be down.

 

[Doc Al]

I always though that the power should be directed up, because m1 tries to slide back, so f_s has to hold it, but in the solution of this problem they wrote the opposite, so I'm pretty confuse.

You are correct. The friction force on m1 must be directed up the incline.