# Bungee Jumping and Conservation of Energy

1. Nov 22, 2007

### cse63146

[SOLVED] Bungee Jumping and Conservation of Energy

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

A bored college student decides to try bungee jumping. He attaches an elastic bungee cord to his ankles and happily jumps off a tall bridge across a river. He ends up barely touching the water before the cord jerks him back up.

Find the mass of the student m.
Express the student's mass in terms of variables given in the problem introduction and any appropriate constants.

2. Relevant equations

mgy = 0.5K$$X^{2}$$

3. The attempt at a solution

so I get m = (0.5K$$X^{2}$$)/gy

but this doesn't seem right. Any ideas?

Last edited: Nov 22, 2007
2. Nov 22, 2007

### Staff: Mentor

What doesn't seem right about it? Looks OK to me. (Not a good model for a real bungee cord, but that's OK.)

3. Nov 22, 2007

### cse63146

would I use $$y_{e}$$ instead of y and X?

4. Nov 22, 2007

### Staff: Mentor

What is $y_{e}$? If that's how much the cord stretches, then you should use that instead of X.

5. Nov 22, 2007

### cse63146

It's there on the picture, but it looks like it only goes half way (y_e) of the bungee cord

I tried inputing this:$$o.5k(y_e)^2/gy_0$$ and it says it's wrong. Any suggestions?

Last edited: Nov 22, 2007
6. Nov 22, 2007

### Staff: Mentor

Without a clear definition, we can only guess. It's not clear from the diagram, but I presume y_e is either the unstretched length of the bungee cord, or the amount that it stretches. There's no additional information? (What text is this?)

7. Nov 22, 2007

### cse63146

It's masteringphysics (online assignment). It's says that y_e is the length the of relaxed cord y_o is the distance between the bridge and the water and y_o>y_e, so once the cord stretches, would it have a length of y_o?

8. Nov 22, 2007

### Staff: Mentor

Yes. Now that you know what y_e and y_0 mean, you can find the amount of stretch in the cord at the lowest point. (Of course, the cord is attached to his ankles, so it doesn't stretch the full distance to y_0. Lacking any data, I would ignore that issue.)

9. Nov 22, 2007

### cse63146

Got it, thanks again.