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If you haven't seen the video of the FlyBoard, please have a look: http://www.youtube.com/watch?v=Cd6C1vIyQ3w&feature=youtube_gdata_player

Yes, it's amazing, but do the physics make sense or is this another "doctored" video?

Specifically, I'm wondering how Newton's 3rd law comes into play here. I'm not an expert in jetskis or physics, but I understand that a jetski sucks in water (not sure from which direction) and expels it out of a rear exhaust. Surely that should create an opposite force on the jetski itself and pushes it forward (regardless of the host that is now attached)? However, in the videos, the jetskis are relatively stationary.

Also, as we know from our garden hoses, when it's full of pressure, it tends to want to "straighten", but these hoses seem to be totally content at being full of pressure but constantly "curved".

Lastly, I think you'll need unparalleled balance to actually bring this off, almost like trying to balance a marble on top of a balloon. But I could be wrong on this.

Your inputs please?

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2 Answers

I am no expert on Newton but I do have a Flyboard and can maybe help explain a few things.

Firstly, the forces on the jetski - it pumps water through the impeller and down the hose so the output force is 10m away on the Flyboard. The only motion you get on the ski is firstly a little backward movement from it sucking the water in, then motion as it is dragged along by the Flyboard.

The reason the pipe doesn't go rigid is the same as your garden hose analogy - or indeed a fire hose which operates on a similar pressure. Neither would stand up on their own, and the flex in the pipe coupled with the movement of the flyboard allow it to bend, twist and give the acrobatic movements you see on the video.

As for balancing a marble on a balloon, if you see the video, you've got three things to take into account. Firstly the pipe provides some rigidity, secondly the two foot jets and two hand jets along with the pipe provide a sort of triangular base. Thirdly the jets of water widen as you go higher, so the 'platform' of water you stand on are in fact two or three feet wide at the base. For sure it is a balancing act, and you've got a lot of chaotic pressure under you but in the same way as we learn to walk, ride a bike or stand on a skateboard, the Flyboard user soon finds a balance and can stand up pretty easily.

And no, it's not a doctored video - simply some well skilled guys taking the kit to the maximum - virtually anyone can do it given a couple of hours and a warm wetsuit :)

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I am confident that the flyboard is a real thing and the video is authentic and unedited.

There's nothing physically impossible about the process. In your objection, you are first confusing yourself by the sentence "jetski sucks in water". An object can't just "spontaneously suck in water". The water is being pumped into an object because the pressure of the water outside exceeds the pressure inside. So a different device, well, a pump, is pumping lots of water into the jetski.

There is such a pump somewhere inside the boats. And indeed, Newton's 3rd law applies over there. Because the pumps are redirecting lots of water "up", there is downward extra force acting on the pump (and therefore the boat). But there's clearly no downward force related to the "sucking" near the daredevil. Quite on the contrary, what he experiences is some residual outward pressure from the hose (or, equivalently, the inertia of the water molecules going up and reflecting from the athlete before they return down, faster than if he were absent) and it drives him up.

You should look at an analogous device, an ordinary fountain, e.g.

http://www.youtube.com/watch?v=w6-jgoqC4FQ

It's clear that the water goes up because of some pumps at the bottom. They will the force from Newton's 3rd law but they can deal with it because they're attached to the ground. If you sit at the top of the fountain (the water), assuming it is strong enough and properly directed you could also "fly" because the water would be pushing you up. The flyboard is analogous except that the pumps are attached to a boat, not quite to the ground, but it's good enough, and except that the water travels through a hose, but that's not a big difference either because the fountain above creates hose-like streams of water, too.

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protected by Qmechanic Mar 7 '13 at 20:28

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