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-   -   How does a trampoline work? (http://www.physicsforums.com/showthread.php?t=653215)

Gigantron Nov18-12 03:55 PM

How does a trampoline work?
 
I was hoping that somebody could explain the physics behind a person bouncing up and down the trampoline. I know the basics as far as what happens to Kinetic Energy, Elastic potential energy, and gravitational potential energy...but there certainly has to be more than this, right? Can someone just explain the physics as to what's happening to an object bouncing on a trampoline on the very bottom, right in the middle, and at the very top?

Freespader Nov18-12 04:05 PM

Re: How does a trampoline work?
 
I think you've got the basics right there, but in case you don't I'll try to explain it really quickly.

When you come down, right before you hit the trampoline, your energy is almost all KE, with only an insignificant amount of GPE. Immediately, when you hit, it acts like a spring, pushing up on you, slowing you down and building up EPE. When you get to the bottom, you have 0 GPE, 0 KE, and all EPE. Then it starts pushing on you. Additionally, you start pushing with your legs (or you were pushing the whole time, it doesn't matter.) Since you were probably scrunched up, straightening out pushes your body up and your legs into the trampoline, which acts on you with a NF, upwards. This increases the upward force, in addition to the "spring" force. Just when you get to the regular height of the trampoline, it stops pushing (theoretically speaking. The trampoline has some mass, so it would continue a little, but not much). Then, you have a tiny amount of GPE and the rest of your energy is KE. When you get to the top of your bounce, all your energy is GPE, no elastic and no KE. Then you start coming down and the cycle restarts again.

Hope that answered your question.

rcgldr Nov18-12 06:23 PM

Re: How does a trampoline work?
 
A trampoline provides a near ellastic collision between the person and the bed (surface) of the trampoline, and also extends the time of the collision, reducing the peak amount of force related to the deceleration and acceleration of each bounce. A person can use his/her muscles to increase the force, and the increase in KE will be equal to the net increase in force over the distance of the bounce. Typically most of the effort is performed near the bottom of the bounce, where the total force is greatest. Note the forces in a competitive situation are large enough that a person can't bend the legs very much without the legs buckling, but only a small amount of relative leg motion is required to maintain or increase KE on a high bounce with high forces involved. Actually it's possible to generate enough force by swinging arms around in a circle at the right moments with the legs kept stiff on a quality trampoline to increase the KE for a fairly high bounce.

Chestermiller Nov20-12 07:56 PM

Re: How does a trampoline work?
 
The trampoline "membrane" is under "in plane" tensile stress (which is initially horizontal). When you exert a downward force on the center of the membrane, the center moves down, and this changes the orientation of the membrane, so that, now the membrane stress has a vertical component. This is the force that the trampolene exerts on you. Take a rubber band, and stretch it horizontally between your fingers. Now push down on the center of the rubber band, and feel the restoring force.


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