# Homework Help: Homework help please! Friction

1. Feb 10, 2010

### jwang023

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

to move a large crate across a rough floor you push it with a force of 21 degrees below the horizontal of F. find the necessary force to move the 32 kg crate with a coefficient of static friction of floor and crate is 0.56?

2. Relevant equations

3. The attempt at a solution

2. Feb 10, 2010

### thebigstar25

please show your attempt to solve the question .. Then we will help ..

3. Feb 10, 2010

### jwang023

Well, I went ahead and made a picture representing what I needed and what not. I than set the x and y directions to try to make equations to solve for the variables.

So, in the x direction, I had -(mu sub k) + Fcos21= ma
For the y direction I set an equation -mg+N-Fsin21=0

I wrote fk =mu sub k*N on the side for reference. I was stuck because I figured I have to find force first right?

4. Feb 11, 2010

### thebigstar25

have you drawn a free body diagram carefully?

In the x-direction the friction force is not(mu sub k) this is not a force?, and another thing you are not given (mu sub k) you are given (mu sub s) dont you think you should use it?

I suggest you to do the free body diagram more carefully this time ..

You know that, Fx(net)=ma and Fy(net)= zero .. You are given the mass m=32kg and (mu sub s) substituting in the equations you will end up with 2 unknowns F (what the question wants) and a (the acceleration) , so do you have to make a certain assumption ? Try to think about it and let us know your answer :) ..

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