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

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The discussion centers on the direction of the static friction force between two masses, m1 and m2, moving up an incline with acceleration. It is clarified that the friction force on m1 acts up the incline to prevent it from sliding back, while m2 experiences an equal and opposite friction force directed down the incline. The confusion arises from differing interpretations of the problem's solution, but it is confirmed that the friction force on m1 is indeed directed up. Understanding the interaction between the two masses is crucial for solving the problem correctly. The key takeaway is that friction forces between the two masses act in opposite directions.
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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.

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The Attempt at a Solution

 

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What's accelerating the blocks up the incline? Is there an applied force? Where is it applied?
 
Yeah, sorry.
Here is a better one.
 

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Much better diagram!

asi123 said:
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.
 
asi123 said:
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.
 

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