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B Is what is depicted in this fake video possible?

  1. Dec 28, 2017 #1
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  3. Dec 28, 2017 #2

    kuruman

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    I'll stick my neck out and say that it is possible to do it. Some mechanical energy is lost from beginning to end due to air resistance and friction at the trampoline which means that the center of mass of the jumper will return to a lower height than it started. The jumpers compensate for this by raising their arms before jumping which maximizes the pre-jump potential energy of the CM. To this, they add more mechanical energy by jumping up and across instead of just stepping off. When they bounce back up, their arms are down and their torso is also bent down which puts their CM lower than when they started, close to the point where it ought to be after losing some mechanical energy. Of course, it takes a very good quality trampoline to pull this off.

    On edit: It also takes some well-trained and athletic people.
     
    Last edited: Dec 28, 2017
  4. Dec 28, 2017 #3

    scottdave

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    I'm not sure how they do it. It looks like it is difficult, but maybe not impossible. I have viewed it several times now. I don't think they are coming up higher than where they "fall" from. They jump up before falling. It appears like they may be tucking in a little when they come back up to lower the center of gravity.
     
  5. Dec 29, 2017 #4

    davenn

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    I don't think it is fake

    @kuruman and @scottdave have both given good reasons as to why this is possible


    so why do YOU think it is fake ?

    likewise .... I agree with your comments

    throughout my multiple views I was looking very carefully at body and arm positions to see if there was just reversal of video of the launch an landing from and back onto the platform
    There clearly is no reversal. Arm, leg and body positions are very different between the two

    Dave
     
  6. Dec 29, 2017 #5

    cjl

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    Looks real to me. Why do you say it's clearly faked?
     
  7. Dec 29, 2017 #6

    russ_watters

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    I guess you haven't seen Cirque du Solei:


     
  8. Dec 29, 2017 #7

    davenn

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    gosh, didn't even think of those dudes as a good example ...... they do amazing stuff
     
  9. Dec 30, 2017 #8
    If the video is fake, it would be reversals of other jumps. Getting enough fitting jumps would be hard work but possible. But sometimes they land with outstretched legs. As I don't thik they could jump like that, I think it is real.
     
  10. Dec 31, 2017 #9

    CWatters

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    If you look carefully you can see they don't land exactly in the middle of the trampoline but slightly on the far side. It's this that causes the rearward force that restores them to the platform.

    I vote real.

    Look up trampoline wall tricks on YouTube for more.
     
  11. Dec 31, 2017 #10

    rcgldr

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    Explanation:

    The trampoline is more like a net than a solid surface, very little air resistance on the trampoline mat, so there's not much loss in energy during a bounce. The old competition canvas type trampolines used 1/4 inch straps with 5/8 inch or larger "holes". Current versions of competition tramps are more net like, often called "ozzie" (Australian) trampolines (although some brands falsely claim to be "ozzie" like).

    During the actual bounce on the trampoline surface, the bouncers are contracting and extending their legs to add energy to each bounce. Leg extension is done near max g force on the trampoline, which is how work is performed to regain energy. The increase in energy from leg movement during a bounce is more the enough to compensate for energy losses.

    Note that the goal in the video is to "stick" the landing, not to maximize height from the bounce, so the bouncers are only trying to add just enough energy (with leg movement during bounce) to have a clean landing back on the platform.

    With these efficient net like trampolines, you can start by lying flat on your back, then use leg motions to initiate movement, and continue to land flat on your back, using leg movement to increase the height of each bounce, eventually reaching a point that a bouncer can start doing a flip between each bounce and continue to gain height until reaching some limit.

    Note that landing off center on competition type trampolines doesn't perceptibly bounce a person back towards the center. It doesn't take any noticeable change in method to bounce vertically while bouncing quite a bit off center, and in the video, some of the bouncers land in the center of the trampoline and still return to the platform. This is because they use the extend their legs a bit backwards during the bounce to end up back on the platform, combined with contacting the trampoline surface a bit short of perfectly flat rotation so that the bouncer "rolls" backwards during the bounce to end up back on the platform.

    This example during a practice shows more variations that what is used during actual performance. At 1:50 into the video, you can see the "reverse" bounce seen in the OP's video. Also you can see the bouncers can't quite return to the top of this particular wall (it's taller than the one in OP's video), but can return to the highest extended ledge. At 2:44 into the video, you can see the net like surface of the trampoline.

     
    Last edited: Jan 1, 2018
  12. Jan 1, 2018 #11

    lekh2003

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    I took it a little further, I just watched the video very closely, in slo-mo. Look at the woman on the trampoline which comes into the video on the right for a couple of seconds. She is doing a normal forward movement on the trampoline. This passes the real test. Initially, it looked preposterously fake, but that's skill.
     
  13. Jan 2, 2018 #12

    A.T.

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    Another way to compensate for energy loss, is to actively push off of the trampoline, by accelerating your limbs, while your back is supported by the sheet. But I don't know if they do that here.
     
  14. Jan 2, 2018 #13

    lekh2003

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    Probably do. They would have to do everything possible to regain their potential energy lost to friction and air resistance.
     
  15. Jan 2, 2018 #14

    rcgldr

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    Which is what I mentioned in post #10. The leg movements you see in the video is how they "push off" the trampoline during a bounce. The ideal is to retract the legs before reaching max acceleration during a bounce, and to extend the legs upwards during the period near maximum force from the trampoline, which increases force and duration, which in turn increases the energy. Also as posted, the trampoline is very efficient at the height they're jumping from, and since the goal is to "stick" the landing back on the platform, the bouncers are only generating enough energy to return back to the platform, rather than trying to increase their height by using stronger leg movements.

    Having experience on trampolines (many years ago), the video in the first post seemed normal to me, and I though the OP's issue was how the bouncers were able to change direction to get back to the platform, as opposed to having enough energy to get back to the platform (the direction change and maintaining energy via leg movement is well known to people familiar with trampolines, although wall jumping was rare back back in the days when I did trampoline).

    In this video, you can see similar leg movement used for doing forward flips off a person's back.



    A variety of wall and trampoline moves, in a few spots in this video, the energy is increased during the bounce. One of the clips in this video has been sped up (not sure why).

     
    Last edited: Jan 3, 2018
  16. Jan 3, 2018 #15

    CWatters

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    At least one maker of trampolines claims to have "self centering technology". I'm not sure how unique a feature is (I thought it was a basic property of most trampolines?) but this is what I was thinking about in #9 when I suggested that they do it by landing slightly on the far side of the trampoline.
     
  17. Jan 3, 2018 #16

    rcgldr

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    This would be a bad idea in the case of competition trampolines, since any "self centering" is going to introduce a torque not initiated by the bouncer, and could over-correct on very high bounces. For competition trampolines, the goal is to eliminate any self centering effect by the trampoline.

    Here is a direct link to the youtube video, at 0:23 into the video, the guy with red shirt lands on the far side of the trampoline, followed by the guy with black shirt at 0:30 that lands in the middle, then both land a bit off center after that.



    Video of the 2017 world trampoline championships. Even though the bouncers "land" off target, the trampoline doesn't induce any self centering effects, which could be an issue at the heights involved. "Landing" outside the red "box" at the center of the trampoline results in deduction, but it doesn't matter how far a landing occurs outside the "box", the deduction is the same.

     
    Last edited: Jan 3, 2018
  18. Jan 4, 2018 #17

    CWatters

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    So when they bounce on their backs are they kicking out with their legs so friction with the trampoline can be used to generate a no-vertical component?
     
  19. Jan 4, 2018 #18

    kuruman

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    I would phrase this in terms of torques at the time of the bounce. Note that the jumpers must make a complete revolution about their CM from beginning to end of the jump. When they hit the trampoline, they have rotated by 90°. At that point their angular momentum will go to zero unless they do something to provide a contact torque and boost their angular momentum so that they can complete the revolution by another 270° upon take off. They achieve this by moving their limbs appropriately and perhaps pushing with their elbows or flexing their body.
     
  20. Jan 4, 2018 #19

    CWatters

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    Ah I see. Bit like the trick you can do with a ball. Give it back spin and you can throw it away from you and it will bounce back.

    So I wonder if you can do a forwards somersault and still land back on the wall?
     
  21. Jan 4, 2018 #20

    rcgldr

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    That would be more difficult since the bouncer can't see the trampoline during much of the front flip before bouncing. If you're wondering if the bouncer could reverse the direction of rotation, I have seen people face away from the trampoline, then do a 1/4 or 1 + 1/4 back flip onto the trampoline to land on their back, and then a 1/4 front flip to return to the platform.
     
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