# Homework Help: Using newton's laws to find an expression for the angle of inclination

1. Sep 16, 2014

### taotao

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

Use Netwon's laws to find an expression for the angle of incilination theta at which the refrigerator starts to slide. Express your answer in terms of possible g, Us, Uk and m.

2. Relevant equations

Fnet=ma
Ff=UFn

3. The attempt at a solution

I attached my attempt at the solution, I would like to know if I did it properly or made a mistake somewhere.

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

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

• ###### photo.JPG
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2. Sep 16, 2014

### BiGyElLoWhAt

You lost a cosine (or just never included it). See if you can figure out where. That's the only thing I see.

3. Sep 16, 2014

### nrqed

The friction force is $\mu_s N$ where N is the normal force. The normal force here is $not$ mg. Do you see why?

4. Sep 16, 2014

### taotao

On the third line: Fg x sin theta - Us x mg x cos theta? =ma

The cosine should be there to account for the magnitude of the normal force right?

5. Sep 16, 2014

### nrqed

That's correct!

6. Sep 16, 2014

### BiGyElLoWhAt

Congrats, good job.

7. Sep 16, 2014

### taotao

For my final answer I get: theta = a/g + mu_s*cos theta
Is there any way to simplify that further?

8. Sep 16, 2014

### nrqed

I don't see how you get a $\theta$ alone, outside a trig function.

Also, you are considering the case when it is just about to start sliding. So what is the acceleration then?

9. Sep 16, 2014

### taotao

So a=0 and it would only be mu_S*costheta