1. Aug 10, 2004

Aly

Grav.

Thanks for all your help guys

Last edited: Aug 16, 2004
2. Aug 10, 2004

Aly

Anyone?

3. Aug 11, 2004

pervect

Staff Emeritus
http://www.nolimits-bungee.com/engineering/acro_e.html
It looks like the manufacturer will rate the cord for load, I'm not sure about the distance jumped.

I didn't look at the guarantee, though, the print was too fine. I'll assume they'll refund the price of the cord if it fails * :-) :-)

*if the buyer requests it in person

4. Aug 11, 2004

NSX

At the top (40m above ground, or whatever it is @ the bottom). you have MAX PE, while the PE of the bungee is 0. At the bottom you have no PE, while the bungee has MAX PE.

Assuming no energy loss, your PE is "mgh", and the bungee's (assuming it is quite elastic) is "(1/2)(kx^2)".

I believe that when they say "solve for the parameter", it means that you're solving for the spring constant of the bungee, "k".

Of course, the above scenario was the simplified version, where you are considered a point mass. You could make calculations a bit more accurate and assume your centre of mass is located at half your height, and implement that information into your calculations.

I think air friction due to you coming down can be neglected.

I don't think the length of the rope matters, as long as there is some rope to begin with.

5. Aug 11, 2004

you cant really calculate a specific k constant for a specific mass. You should make a constant that will have a range for different masses, ie. 60kg - 120kg. And you should end up with a rage of streches for the rope. Also, you have to factor in the length of the rope when you are dealing with the equation:

$$P_e = \frac{1}{2}kx^2$$

6. Aug 11, 2004

pervect

Staff Emeritus
Just to expand on this a little more, the idea is to have a k "stiff enough" that the rope won't break, or stretch permanently (plastic flow). I presume that the ropes are rated to stretch to some fraction of their original length, but I don't know what that fraction is.

The good news is that apparrently when using a stretchy rope one doesn't need the same LARGE safety factors that one needs when trying to suspend a person from a static rope. The stretch apparently takes care of the dynamic loading issue that makes the large safety factors necessary when using a static (non-stretching) rope.

7. Aug 12, 2004

Staff: Mentor

1. I wouldn't bother with it (unless you were told to research it) - make your own rope. How about one that starts at 15m long and stretches to 30?

2. Length of rope matters, but since it wasn't given to you, you get to pick. Longer rope means less stretch (longer freefall, more of a jerk at the end) and a shorter rope means more stretch.

3. Not good or bad, just information. A 15m rope stretching to 30 would fit that bill.