# Conservation of mass question

1. Nov 25, 2012

### BlueCardBird

1. The problem statement, all variables and given/known data
A bungee jumper of mass 75 kg is standing on a platform 53m above the river. The length of the unstretched bungee cord is 11m. The spring constant of the cord is 65.5N/m. Calculate the jumpers speed at 19m below the bridge on the first fall.

2. Relevant equations
Em1=Em2 Ek=1/2mv^2 Es=1/2kx^2 GPE=mgh

3. The attempt at a solution

Find x by subtracting 11m from 19m which is 8m, this is how far the bungee cord is stretched. Use x in formula gpe=Ek+Es. Problem arises when I sub values in answer is off.

2. Nov 25, 2012

### Staff: Mentor

Show us what you did.

3. Nov 25, 2012

### BlueCardBird

x=19m-11m
mgh=1/2mv^2+1/2kx^2
subbed values in but didnt get right answer :S

4. Nov 25, 2012

### Staff: Mentor

What did you use for h?

5. Nov 25, 2012

### BlueCardBird

Height of the cliff or bridge which is 53m

6. Nov 25, 2012

### Staff: Mentor

That's your problem. He hasn't fallen the whole distance yet. He has only fallen h = 19 m.

7. Nov 25, 2012

### BlueCardBird

wait but total energy at top of the cliff is mgh where height is 53 m though

8. Nov 25, 2012

### Staff: Mentor

Try 19 m in your equations and see what you get. The equation should be mgΔh. It is the change in potential energy that matters, not the total potential energy relative to some arbitrary reference.