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# Physics Help!?

You jump off a platform 134m above a river, while attached to a bungee cord with an unstretched length of 40 m. You then fall another 80 m before the bungee cord, which obeys Hooke's Law, brings you to a stop and then starts pulling you upward. What is your net acceleration in units of g (9.81 m/s^2) due to the bungee cord and gravity at the lowest point where your velocity is zero?

Can someone please explain how to get this answer? Thank you!

### 1 Answer

- NCSLv 72 years agoFavorite Answer
GPE becomes SPE

mgh = ½kx²

so you need the mass m to proceed.

My mass is around 80 kg:

80kg * 9.8m/s² * 120m = ½ * k * (80m)²

so

k = 29.4 kg/s²

At maximum extension, then,

net F = kx - mg = 29.4kg/s² * 80m - 80kg * 9.8m/s² = 1568 N

so acceleration

a = F / m = 1568N / 80kg = 19.6 m/s² = 2*g

and I suspect that this is true regardless of your mass.

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