# Gravity Potential in a Bungee

1. Mar 10, 2014

### oppyykm

If the person has not yet jumped, is their GPE equal to mgh, where h is the height above the ground, or mgL, the maximum length the bungee rope can stretch?
Thanks

2. Mar 10, 2014

### davenn

Hi oppyykm
welcome to PF

which do you think is correct and why ?

Dave

3. Mar 11, 2014

### oppyykm

I thought it would be the maximum length the bungee rope can stretch, but i'm not really sure. Because if its maximum length is shorter than the height, it doesn't have the potential to fall the full height ever, so therefore it can't have potential equal to height.

4. Mar 11, 2014

### jbriggs444

What final result is it that you are trying to calculate? The length to which the bungee ends up stretching?

I would recommend that you perform that calculation three times: Once with the assumption that potential energy is given by mgh where h is measured from the ground under the bridge. Once with the assumption that potential energy is given by mgh where h is measured from the deck of the bridge (and will be negative throughout the drop). Once with the assumption that potential energy is given by mgh where h is measured from the maximum length of the bungee.

Compare to see if the choice makes any difference in the calculated value. Do you think that it will?

5. Mar 11, 2014

### Staff: Mentor

GPE is always equal to mgh, but you are free to define h=0 wherever you like as long as you stay consistent. I would highly recommend doing the exercise jbriggs444 suggested!

6. Mar 11, 2014

### Staff: Mentor

All that ever matters in the end is the difference in PE between two locations, which is what I think jbriggs444 was trying to get at. No matter where you define h = 0 (giving PE = 0 at that location), the difference in PE between (for example) the ground and the bridge deck is the same.