1more physics questions?

Question

each individual bungee chord has an elastic constant fo 200 N/m and is 35 m long.A jumper will use a number of these to safely leap from a bridge which is 45 m above a river. (two chords double K, three triple etc) The jumper is 88 kg and we measure h=0 at the point where the chords snap taut. How many chords does she need if she wants to just touch the surface of the water?

On the back, he wrote the answer as 3 but didn't put an explanation.

I tried all of the equations but couldn't figure it out...HELP!!!

Answer 1

If I read the question correctly, the point where the chords snap taut is 10m above the water. That means that the chords must absorb all of the kinetic energy of the jumper in exactly 10m.

The kinetic energy of the jumper will be equal to the potential energy lost in the jump minus the energy stored in the chords.
m*g*h=N*1/2*k*x^2
where
m=88
g=9.81
h=45
N= number of chords
k=200
x=10
m*g*h*2/(k*x^2)=N
=88*9.81*45*2/(200*100)

I get 3.9
So use 4 and she will almost touch the water.

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