# Little needed with homework

1. Oct 29, 2004

### nazarip

A 30.3 N child is in a swing that is attached to ropes 2.14 m long. What is the gravitational potential energy when the ropes make a 30.9° angle with the vertical?

So far ive tried 6 different answers ranging between 0-64.8 J and no luck. Need a clue to get me on the right track. Thanks in advance.

2. Oct 29, 2004

### Sirus

You know $E_{gravitational potential}=mgh$, where h is height off the ground and m is mass. I believe you must assume that when the ropes make an angle of 0 with the vertical h=0. Does that help?

3. Oct 29, 2004

### JasonRox

Pretend that its like those incline questions.

4. Oct 29, 2004

### nazarip

I did assume that when the angle is zero E=0 and when the angle is 90 E=mgh=64.8, so I thought the answer would be (sin30.9)(mgh), but that answer was incorrect. I also tried (sin59.1)(mgh) which was also incorrect, Im not sure where to go from here.

5. Oct 29, 2004

### Corneo

Well I think when the kid swings up to make a $30.9^\circ$ with the vertical, his height above the ground is increased. By how much? The answer is

$$R \cos (30.9^\circ)$$

So the kid's $U=mg(R\cos (30.8^\circ))$