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B How much force would it take to jump on air?

  1. Apr 15, 2017 #1
    What i mean is how much force would a person lets say 160lbs have to kick the air it be propelled upwards?

    This is all based around the double jump idea from games... Air is a gas and gases have resistance so it makes scene if you could put enough force in to a spot you should be able to be propelled up...
     
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  3. Apr 15, 2017 #2

    A.T.

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    More than 160lbs.
     
  4. Apr 15, 2017 #3

    sophiecentaur

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    The trite (smartarse Physics teacher) answer is that the force would need to be just over their weight force. The question is more, how one could generate sufficient pressure under your foot by kicking downwards. The answer to that is that it would be impracticable because the foot would need to be travelling downwards at a velocity* that the muscles just couldn't achieve. You would need some sort of wing arrangement (with a large area) that would shift enough volume of air to generate a temporary increase in pressure (flapping?)
    As an idea of the necessary velocity* consider that a skydiver balances her weight at a terminal velocity of around 50m/s, using the whole area of her spread body. Multiply that by a factor of at least 10 for one foot 's worth of area (500m/s) and you are well over the speed of sound (340m/s).
    Apologies to A.T. - I cross posted. The smartarse I was referring to was me!
     
  5. Apr 15, 2017 #4

    berkeman

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  6. Apr 15, 2017 #5

    Nugatory

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    Think about the simpler case of a 160lb person jumping off the floor. Gravity is pulling the person down with a force of 160 lbs. If the floor is pushing on the person's feet with an upwards force of more than 160 lbs, the person will be thrown into the air. (If the force from the floor is exactly equal to 160 lbs, which is the most common situation, the person will just be standing there, and if the force is less the floor is unable to support the weight and the person is falling).

    So to jump off the floor, the person has to do something to make the floor exert an upwards force of more than 160 lbs. Now you need Newton's third law, the one about equal and opposite forces: when the person exerts a force of more than 160 lbs on the floor, the floor will exert a force of more than 160 ls on the person, and the person will be thrown into the air. The person starts with a downwards force of 160 lbs from gravity, and then by pushing down hard with their leg muscles they increase the force they're exerting on the floor, the force exerted by the floor on the person increases, and up into the air they go.

    So now let's move on to your double jump problem. Somehow, we have to exert a force of more than 160 lbs on the air underneath the person's feet so that air will exert an equal and opposite force on the person's feet. But it's not easy to do that, because the air is so light and accelerates so quickly under even very small forces that we can't push on it that hard - if we try it just moves out of the way instead of pushing back. We would need to moving our feet downwards very quickly indeed, as described in @sophiecentaur's answer above.
     
    Last edited: Apr 15, 2017
  7. Apr 15, 2017 #6

    sophiecentaur

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  8. Apr 15, 2017 #7

    phinds

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    No it doesn't. Before you got anywhere near moving your legs fast enough you would likely have set your inner thighs on fire.
     
  9. Jun 23, 2017 #8
    I think matpat did a video about Genji's double jump in Overwatch.

    tldw: you would essentially need the equivalent of a small nuclear bomb detonating under your feet. More if you're falling down and less if you still retain upwards momentum.

    Edit: ah, I seem to be too late.
     
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