Conservation of Energy - Bungee Jumping

[jtw2e]

Conservation of Energy -- Bungee Jumping

Homework Statement

Unstretched length = 139 ft
k = 39 lb/ft
Weight of jumper = 122 lbs

Homework Equations

Assuming he hasn't hit the ground, how much has the bungee chord stretched when the professor is at the bottom of the jump? (ft)

The Attempt at a Solution

I attempted to setup a CoE equation narrowing it down to:
KE₁ = U_elas2

I came up with ~29.5 ft of stretch in the rope. This was wrong.

Does the jumper have some potential energy as he is falling? I cannot see why I'm getting this one wrong. Thanks for any help you could offer.

 

[SammyS]

Where is the fixed end of the cord relative to the professor's location at his jumping off point?

 

[jtw2e]

Where is the fixed end of the cord relative to the professor's location at his jumping off point?

Well, I guess the fixed end would be attached to the bridge at the point where he jumped off. But I chose x (for Kx1) to be at the position where the rope is neither stretched nor unstretched; i.e. 139 ft.

I found his velocity at the time the rope begins to stretch to be 94.6 ft/s which was correct, but I still am having trouble finding how much the rope stretched at it's maximum point before it started to pull him upwards.

Should it be KE₁ + U_elas1 = U_elas2 ?

 

[SammyS]

Since you already calculated KE, you don't need mgh for the 139ft.

How ever you need to include -mgx as a loss of potential energy as the cord is stretched.