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A great fall

  1. Sep 13, 2008 #1
    Ok lets just say you're falling from the top of the sky scraper but standing on a car.
    Your weight is around 60kg and the car weighs around 1 tonne. (1000kg)
    Assuming that you don't die before you hit the ground, is it possible to jump off the car, say when your three meters from the ground and survive?
    Theres a diagram below if you can't picture what im trying to say
    !=1, was a typo sorry bout that
    [​IMG]
     
  2. jcsd
  3. Sep 13, 2008 #2

    madmike159

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    Gold Member

    For this question I'm ignoring air resistance.
    Well to work it out you would work out your speed you would use Newtons equasions. Say the car had mass 1000Kg and you had mass 60Kg. You would both have acceleration of 9.81ms^-1.
    Using V^2=u^2+2aS your velocity 2m before you hit the ground would be 140ms^-1
    using p=mv the cars momentum would be 140000 Kgms^-1 and yours would be 8400Kgms^-1.

    So no you couldn't jump off because you could not creat enough force to stop your self. Even if you could the chances of timing it right are very small.
     
  4. Sep 13, 2008 #3

    LURCH

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    Just to add to what has already been said:

    It is true that you could never generate anything ressembling the acceleration needed to save you in a fall from a very tall building. However, there must be some height at which your landing would be just barely fast enough so that your legs cannot absorb the shock and save your life. From this height, it should be posiible to use this trick to slow your landing speed from just barely above this critical speed, to just barely below it, saving yuor life. In eral life, this very precise set of condition will never arise, and the trick will not save you. But, if I find myself in a fall and there is something to puch off from, I am certainly going to try it!
     
  5. Sep 13, 2008 #4
    I should of added in that i'm in year 9, so i don't understand your equation but i guess its basically saying that the gravity pushing down on me is to hard to resist and that i couldnt jump off?
     
  6. Sep 13, 2008 #5

    russ_watters

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    Staff: Mentor

    It isn't about the gravitational force, it is about the speed of the falling car vs the speed of the jumping person.

    Think about it this way: A person who is capable of a vertical leap of 4 feet gives him/herself a velocity of 16 feet per second (about 10 mph). Terminal velocity of a car is on the order of 100 miles per hour. So if you time your jump perfectly, you'll still hit the ground a 100-10=90 miles per hour and still be killed.
     
  7. Sep 14, 2008 #6

    madmike159

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    Sorry got a bit carried away. Most of it isn't even needed...

    but basically what we're etting at is it wont work so don't try it:biggrin:
     
  8. Sep 14, 2008 #7

    Dale

    Staff: Mentor

    Your better bet would be to get into the car, buckle in, and hope that the collision is "just right".
     
  9. Sep 14, 2008 #8
    So, you think that somehow your legs could generate enough force to counteract the momentum you've built up during the fall??
    If that's the case then you could probably "leap tall buildings in a single bound!"
     
  10. Sep 14, 2008 #9
    The force required by his legs to jump would be equal to the force he would experience if he hit the ground without jumping. It's like he's standing still on the ground and this car erupts from below and crashes into him. In either case, as he's thinking which reference frame to apply, his leg bones will still break into and crush his upper body.

    So when standing on any car roof, be very certain it is not going to fall of a building or cliff. Those kinds of things don't end well.

    I've also heard this about falling elevators. In that case you die as your head crashes into the ceiling instead of the floor crashing into your feet.

    As Sancho Panchez said to Don Quixote, "it doesn't matter whether the stone hits the pitcher or the pitcher hits the stone, it's going to be bad news for the pitcher."
     
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