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My IA is about bungee jumping where I increase the mass and look at the efficiency of energy at the lowest point of the jump.

My question is: How do I find the spring constant k for an elastic cord? I need it so I can plug it into the elastic potential energy formula.

My thoughts:

I was thinking to do this by mgh=1/2kx^2

but by doing this I get a different k value each time as the mass and therefore the extension, x is changing but isn't k always a constant and cannot change unless the cord deforms... so is this wrong?

I was also wondering if I can find the the k constant using Hooke's law (F=kx). I was thinking to graph force (mg) on the Y-axis and the extension on the X-axis and the slope of that would be the k constant. Will this be a valid approach?

Please help me!

Thanks a looooot in advance!