Running through some random YouTube videos I ran across this one: - YouTube
It’s a high school student doing a huge standing vertical jump. The interesting bit (to me anyway) was the he apparently paused unrealistically at the apex of his jump. This might be way he waves his arms. Or it just longer because the film is slowed down. Or maybe this is a normal jump dynamic.
Does he hang longer than normal, what say the teemings? Reminds me of another thread from years gone by.
Ok, so you see how as his feet get to the top of their rise his arms are at the bottom of their circle, and the hover lasts exactly the amount of time his arms take to do one circle?
That’s the secret.
As his feet are still and his arms are travelling up, his center of gravity is still travelling up slightly. The top of his jump is not the point at which his feet stop moving, it’s the point after that when his arms are pointing straight up.
After that point, he’s travelling downwards (that is, his center of gravity is) but you don’t see it in the feet until his arms have come all the way to the bottom of their circle.
It’s extraordinarily well-timed, but I don’t think it’s defying the law of gravity
It’s an illusion, on this board we obey the fundamental laws of physics.
You take off at a given velocity, you reach a certain height, you stay in the air for a certain time. No technique in the world will give you more time at that maximum height.
Of course it’s an illusion (the OP even guesses at the likely mechanism for it); the question is asking to explain the illusion not whether laws of physics are being defied or not.
You can see in othervideos that he makes several rotations of his arms, when one might expect only one rotation if it was purely a matter of center of gravity. Could it be that, at the moment the upward force and downward force are in balance, the propulsion of the rotating arms means that he is in effect flying for a split-second or so?
Those videos show the same effect as the first one - the amount of time his feet are stationary at the top of the rise is exactly equal to one arm rotation. The fact that he’s done multiple rotations on the way up doesn’t affect that aspect.
What the reason for the continuous arm circles in the first place is, I don’t know, but I presume something to do with either the aerodynamics or biodynamics of the movement. As you say, if he’s just trying to get a good center of gravity for the top of the rise, all he needs to do is to make sure his arms are pointed straight down at that point, the multiple circles aren’t really necessary from a simplistic model-the-elephant-as-a-perfect-sphere-physics point of view. And he’s probably not doing it *just *to get the cool hover effect, though it sure is cool.
We try to fight ignorance on this board, in the thread to that point it hadn’t been clearly and unequivocally stated that it is an illusion and nothing else.
The question did not ask for the* illusion* to be explained and does clearly ask in the title whether it is possible to hover. It also asks…
Aspidistra is correct - he isn’t hovering, which isn’t possible. His center of mass shifts up because his arms are going up, then his center of mass moves lower as his arms come down. The fact that his feet are relatively stationary doesn’t mean his center of mass has stopped moving.
He would only be able to hover if there were some source of energy to push him while he was in the air. Helicopters can hover because their propellers are pushing down on the air. Hummingbirds because their wings are pushing. Jumping is an initial surge of energy from pushing on the ground, and there is no further energy input.
If he were a hummingbird, he could hover by flapping his arms. But the jumper’s mass is far too great, and the surface area of his arms far too small, and the flapping far too slow, to generate any lift worth noting.
Cite.
You are being unnecessarily pendantic. There any number of videos on YouTube showing people jumping up and coming down. Even in slow motion there is not the pause at the apex as noted in the OP linked video.
If your pendanticry has issues with my use of the term hover just say so and move on. Unless you have proof that the arc of this particular jump is the same as every other jump.
Just showing up in this thread and pronouncing “no” isn’t helpful.
seriously? cite for what? the laws of motion? a treatise on ballistics?
There is no pause at the apex, that is not possible and when his jumps are played at normal speed it is entirely consistent with a person who jumps really high. A combination of camera position, frame rate, body movement and unfamiliarity with people jumping really high with that particular body position all contribute to it looking like there is a pause, there is no pause…repeat…no pause
The arc of this jump is fundamentally the same as every single jump ever. It cannot be otherwise. That you even think it could be different is part of the reason why I stated a clear “no”
but it is more accurate than using the words “hover” and “pause” which are entirely unhelpful, inaccurate and misleading.
It reminds me of the “invisible box” illusion somewhat.
(Just watch the beginning of the video.)
The girl is hopping on one foot but by keeping the other foot stationary in the air it looks like she’s stepping on nothing.
The similarity is that in both cases you’re paying attention to the part of the body that stays still while ignoring the parts of the body still moving. That gives the illusion that gravity is being defied.