# Forces problem

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1. Mar 30, 2016

### Balsam

1. The problem statement, all variables and given/known data
You are pulling a 39kg box on a level floor by a rope attached to the box. The rope makes an angle of 21 degrees with the horizontal. The coefficient of kinetic frictions between the box and the floor is 0.23. Calculate the magnitude of the tension in the rope needed to keep the box moving at a constant velocity?

2. Relevant equations
fnet=ma

3. The attempt at a solution

I have pictures of my attempted solution attached. I stopped solving it because I realized that my Ft variables would cancel out and I would have nothing to solve for. Also, what I did makes no sense but I was trying to solve it like you solve other forces problems where you find set up your equations and find a variable present in each equation and set those equations equal to each other. I don't think that works here. My steps are numbered. The attached picture is the first one and the picture at the top is the second one.

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2. Mar 30, 2016

### CWatters

I agree with your starting equation for the vertical sum of forces but I think you have a sine error in the horizontal equation. You appear to have friction acting in the same direction as the tension force (eg both positive).

3. Mar 30, 2016

### Balsam

Is there another way to solve this problem?

4. Mar 30, 2016

### CWatters

I think you are on the right track. I don't think FT will cancel.

PS: it's midnight where I live so I'm off to bed.

5. Mar 30, 2016

### haruspex

6. Mar 31, 2016

### CWatters

+1

Sorry yes I meant sign not sine.