# Homework Help: Rope Swing angle

1. Oct 6, 2008

### ganondorf29

1. The problem statement, all variables and given/known data
The rope of a swing is 2.90 m long. Calculate the angle from the vertical at which a 82.0 kg man must begin to swing in order to have the same KE at the bottom as a 1450 kg car moving at 1.11 m/s (2.48 mph).

2. Relevant equations
k=(m*v^2) / 2
U = mgh

3. The attempt at a solution

I first found the kinetic energy of the car. 1/2*[(1450)*(1.11^2) = kcar. I found kcar to be 893.273 J. Than I used U=mgh to find the height the man must be to have the same amount of kinetic energy. Kcar = 82*9.8*h. I found the height to be 1.11m. Than I set up a triangle and tried cos^-1 = (1.11/2.9) to find theta. I got 67.5 deg, but thats wrong. Any suggestions?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 6, 2008

### phyguy321

How do you know thats wrong? Do you have the answer? Is it 22.5?

3. Oct 6, 2008

### Hootenanny

Staff Emeritus
You're very nearly there, just one small mistake. Does his height really make up one of the sides of the triangle that you need?

4. Oct 6, 2008

Try again

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5. Oct 6, 2008

### ganondorf29

I understand what I did wrong now, but how do I apply that to fix them problem?

6. Oct 6, 2008

### Hootenanny

Staff Emeritus
Well if the height isn't the adjacent side of the triangle, then what is? Look at the sketch that phyguy provided.

7. Oct 6, 2011

### philsfan

I am actually working on a problem very similar to this one and I found the height of the person, but I am stuck on how you find the angle. I looked at the image given in this thread but it was the same image I had already drawn out in my attempt at solving the problem. My thinking is that if the energy is conserved then at the bottom of the swing there would not be any potential energy, it would be all kinetic, implying that the height at the bottom of the swing would be zero. Therefore the adjacent side of the large triangle would be the length of the rope, but I am not sure how you find the hypotenuse of the triangle when you know the height and the length of the rope. I would appreciate help regarding this question, thanks!