Bungee chord (standing waves and antinodes.)

[j88k]

Homework Statement

A 75 g bungee cord has an equilibrium length of 1.20 m. The cord is stretched to a length of 1.80 m, then vibrated at 20 Hz. This produces a standing wave with two antinodes.


What is the spring constant of the bungee cord?

Homework Equations

f=nv/2L

The Attempt at a Solution

Two anti-nodes mean 3 harmonics
f = 20Hz
n = number of harmonics
L = length
v = velocity
20 = nv/(2L)=3v/(2L)
v = 20/3*2*1.8 =24m/s

F = tension
v^2 = F/(linear mass density)
F = 24^2 * 0.075/1.8
F = 24N
F = kx
k =F/x
k = 24/(1.8-1.2) = 40N/m
i
It doesn't seem to be right though!

 

[turin]

I didn't check your solution numerically, but I agree 100% with your approach.