Double Bounce of two balls on a Trampoline

[Physics0009]

Hi--
I'm doing an end of year project on the physics of trampolines. Anyone who has ever trampolined before knows that you can "double bounce" a person, such that one person bounces right before the other, thus launching the second bouncer higher into the air. In order to do a physics experiment with this, with a high res camera, I videotaped two balls (both 6 lbs), dropped such that one double bounces the other higher into the air, while the other ball barely bounces at all. Can anyone help me explain the physics of this 'phenomena' with equations, and/or as much detail as possible...Thank you.

To clarify what I'm talking about specifically watch this video

although it is two kids, instead of two balls, the idea is the same. Help would be greatly appreciated. Thanks again.

 

[rcgldr]

In the case of the timed double bounce, the second person lands onto an already downwards displaced trampoline, displacing it further downwards, which also reduces the updwards force being applied to the first person that landed because of the further downwards displacment. As the trampoline returns back upwards, it is pushing both people back up, but the second person experiences more of the force and for a longer period of time, so ends up bouncing higher. I'm not sure of the ideal timing, as experienced traompolinists do not double bounce using this method.

Although double bouncing can be done by timing, it's more effective for the trampolinists to all land at the same time, then for all but one of them to pull up their legs so that all of them push the trampoline down, but only one of them receives the upwards force. Typically only one person is bouncing high, and the others are barely bouncing, using some amount of momentum and their leg muscles to push the trampoline down at the same time the bouncer lands on the trampoline. It's also more gentle to the person being bounced. When bouncing someone up high, it's safest if the person being bounced is landing on their stomach and doing 1 or 2 back flips back onto their stomach repeatedly (it's called a "cody"). The reason why this is safest is because the center of mass is very close to the surface of the trampoline, so the bounce remains vertical with no potential of going off the trampoline at an angle. The next to last picture on this web page shows this:

http://jeffareid.net/gym.htm

 

[Physics0009]

Thank you! Can you give me any relevant equations to this situation? And, how does conservation of energy and/or conservation of momentum come into play?