# Homework Help: Hard complex reasoning help

1. Sep 13, 2008

### pat666

hard complex reasoning help!!

1. The problem statement, all variables and given/known data
American advertising agency McKAnn decided to bungee jump a car off a bridge to show how well made the car was. They used a 30 m m rubber bungee cord used by the army to bungee tanks during air drops. it had nine individual cords each around 12cm in diameter. when released from the top of the bridge , 267m above the water , the 1857kg ute stretched the cord 6* its original length. Calculate the spring constant of the cord. (k)

2. Relevant equations
F=-kx

forces and spring equations.

3. The attempt at a solution
I have found the area of the bungee to be .135717m^2, also i can calculate the GPE but that is as far as i have gotten- please help. the answer is 254 N M

2. Sep 14, 2008

### LowlyPion

Re: hard complex reasoning help!!

How long is the cord? 30 m?

You might want to consider the change in potential energy from the falling car mg*h as being offset by the potential energy to stop the car at 6 times the length of the cord. How do you calculate the stored potential energy in a spring? The work required to stretch the spring? And work is what? Force over a distance? And for a spring that would be 1/2 k x2? And change in potential energy is also work? Hmmm... sound like a possibility?

Last edited: Sep 14, 2008
3. Sep 14, 2008

### pat666

Re: hard complex reasoning help!!

Yah i will have ago at that now, would one of you mind doing the question step by step for me i have a tone of questions like this and am not really sure how to go about it.

4. Sep 14, 2008

### Defennder

Re: hard complex reasoning help!!

Unfortunately, he cannot do any part of the question for you as per PF rules. Just tell us what you don't understand and we'll try to help you.

5. Sep 14, 2008

### pat666

Re: hard complex reasoning help!!

Dont worry I figured it out from what he told me. thanks
What a stupid rule not being able to show how the question is done.

6. Sep 14, 2008

### Staff: Mentor

Re: hard complex reasoning help!!

You see, you were given a hint and you did the question by yourself.

That's what this rule is for - to force lazy people to do their work with as little help as they need. As proven above, it works.

7. Sep 14, 2008

### pat666

Re: hard complex reasoning help!!

yeah i suppose you do understand it more if you do it yourself. Thanks again

8. Sep 14, 2008

### LowlyPion

Re: hard complex reasoning help!!

That would be the idea then.

Good luck.