Solve Physics Homework: Bunjee Jump Question

[PhysicsTurkey]

Homework Statement

A Year 12 Physics student has a mass of 85kg and is planning to bungee jump from a 200m high tower with a Killer Python Snake strapped to his ankles. The student decides to place his life in your hands and will totally rely on your knowledge of physics.

He requests you to calculate the appropriate cross-sectional diameter of a 50m long Killer Python Snake, which is suitable to safely stop at least 20m above the ground below.

Homework Equations

Young's Modulus = 4854.37Nm^-2
Strain = (Change in length)/(original length)
Energy Absorbed = ((1/2)E.(ε)^2) x Volume
Volume of a cylinder = (pi(r)^2 . L) r= radius, L = Length

The Attempt at a Solution

Here's my attempt :S

GPE = MGH
(85)(10)(180) = 153 000j

Therefore the killer python must absorb at least 153 000j of energy before the student descends 180m. So i set energy absorbed ≥ 153 000

and use equation:
153 000 = ((1/2) E.(ε)^2). (pi (r)^2 .L) where E= young's modulus, ε = strain.
153 000 = ((1/2) (4854.37).(2.6)^2)).(πr^2 .180)

Solving for “r” gives:
r ≥ 0.13Am i on the right track or is this wrong?
Also, should i use length as 180 or 50 when calculating the volume?

Cheers, Jay

 

[PhysicsTurkey]

Any help or another method to go about this would be much appreciated.

 

[PhysicsTurkey]

Is anyone able to help with this?