# Homework Help: What is the direction of the force, up or down?

1. Oct 21, 2008

### asi123

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### scan0001.jpg
File size:
26.1 KB
Views:
76
2. Oct 21, 2008

### Staff: Mentor

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

3. Oct 21, 2008

### asi123

Yeah, sorry.
Here is a better one.

#### Attached Files:

• ###### scan0001.jpg
File size:
12.4 KB
Views:
82
4. Oct 21, 2008

### Staff: Mentor

Much better diagram!

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.

5. Oct 21, 2008

### Staff: Mentor

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