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j88k

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## 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!