Bungee Cord Force Constant from Work?

[CandyApples]

Homework Statement

A bungee jumper jumps with a mass of 61kg jumps off of a bridge. The massless bungee cord is 25m long and beyond that length acts as a spring. The jumper reaches the lowest point 42.9m below the bridge. Find the force constant which characterizes the bungee cord.

Homework Equations

W = F X d
F = -kx

The Attempt at a Solution

First I calculated the work for mg.
Wmg = 42.9 * 61(9.8) = 25646J

Since the object stops, the object has a net work of zero. So:
-25646 = 17.9 * F
F = -1433N

-1433 = -k(17.9)
k = 80N/m.

This seems far too low, could someone please tell me where my logic is flawed? I think it might be In the assumption that balancing net work accounts for the object stopping but i am not sure.

Thanks in advance!

 

[ehild]

The force exerted by a spring is not constant, as you assumed.

ehild