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

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Homework Help Overview

The problem involves two masses, m1 and m2, moving up an incline with acceleration. The original poster is confused about the direction of the static friction force between the masses, questioning whether it acts up or down the incline.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the forces acting on the masses, particularly the role of static friction and its direction. Questions arise regarding the applied forces and their locations, as well as the interaction between the two masses.

Discussion Status

The discussion is active, with participants providing insights into the forces at play. There is recognition that the direction of the friction force depends on which mass is being considered, and some guidance has been offered regarding the nature of the forces involved.

Contextual Notes

There is some confusion regarding the solution provided in the problem, as it appears to contradict the original poster's understanding of the friction force's direction. This indicates a potential misunderstanding of the setup or assumptions involved.

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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.

Homework Equations





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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