Conservation of energy, bungee jumping

[asdfg]

Homework Statement

A daring friend of yours talks you into bungee jumping from the Royal Gorge Bridge, in Colorado. Starting from the bridge (at rest), you launch off and fall first to the unstretched length of the bungee, 153 m. The bungee then stretches until you reach the full drop distance of 321 m. For this problem, approximate your mass as 70 kg.
a. What is the energy stored (or lost) by the bungee?
b. After you drop the full distance, you bounce back up to 50 m below the launch point. How much energy was lost (to air friction and heat) in the first drop?

Homework Equations

PE=mgh

The Attempt at a Solution

Part A) mgh = 70*9.8*321 lost
Part B) 70*9.8*321 - 70*9.8*(321-50)

 

[BvU]

Hello asdfg (keyboard row 2, but you're not the first one !) welcome to PF !

Nice first post: template and all. Is there a question you need help with ?

 

[haruspex]

I don't think you've read the question quite right in part a. It doesn't ask what energy was lost by you or the you-Earth system. It asks what energy was gained (or lost if lost) by the bungee cords.

The question could be worded better in part b. It asks for the energy lost to friction etc. during the drop. You have calculated the energy lost over the whole trip, which I would guess is what was intended.

 

[BvU]

@haruspex : could you clarify ? I also interpreted this as "What was the maximum energy stored in the cord" and agreed with keyboardrow2 A) answer...

 

[haruspex]

@haruspex : could you calrify ? I also interpreted this as "What was the maximum energy stored in the cord" and agreed with keyboardrow2 A) answer...

The answer in the OP says "lost".

 

[BvU]

I see what you mean. Does make it a rather difficult exercise (for me at least, so probably aslo for keyboard !), unless you make some daring assumptions.