# Bungy jump problem

1. May 22, 2010

### ff93

You go bungee jumping from a bridge that is 65m high, the bungee cord has an unstretched length of 32m and a spring constant of k=107 N/m
You have a mass of 75kg.

Question: what is the acceleration at the bottom, closest to the ground?

I`ve tried to use 1/2kA2=1/2mx+1/2kx2
but it doesn´t seem to work for me :(

2. May 22, 2010

### Staff: Mentor

Start by figuring out where the lowest point is. Use conservation of energy.

3. May 22, 2010

### ff93

should I use mgh=1/2kx2?

x as how far it stretches from 32m?

and then should I use F=-kx => F=ma and solve out a?

4. May 22, 2010

### Staff: Mentor

Yes. Hint: Express h in terms of x. (Measure gravitational PE from the lowest point.)

5. May 22, 2010

### ff93

you mean mg(x+32)=1/2kx2?

should I then continue with F=-kx => F=ma solve out a?

6. May 22, 2010

### Staff: Mentor

Exactly.
Be sure to include all forces when applying F=ma.

7. May 22, 2010

### ff93

mg(x+32)=1/2kx2 gives two answers but I can throw away the negative one, right?

then I used F=-kx+mg => a= (-kx+mg)/m
is that right?

Last edited: May 22, 2010
8. May 22, 2010

### Staff: Mentor

Right.

Yes, that's fine.

Don't get hung up with signs. You have an upward force of magnitude kx and downward force of magnitude mg. Which direction will the acceleration be in at the lowest point?

9. May 22, 2010

### ff93

the direction will be "up",negative
right?

10. May 22, 2010

### Staff: Mentor

Yes, the acceleration will be up. Which you happen to call negative. I suspect that they only care about the magnitude of that acceleration.

11. May 22, 2010

### ff93

thank you for the help!