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A question - could you survive this?

  1. Jul 15, 2003 #1
    ok, an argument erupted in the middle of work a few days ago. A huge discussion on whether my extreme sport would actually "work", without killing anyone. So I’m on here to ask you brainy people if or otherwise, the following can work.

    I think this will be more horrifying than amazing than anything else, but sounds like a laugh if you’re on so many drugs your actually brave enough to attempt it. :wink:

    I was thinking (in a highly implausible scene) - You start many miles above the earth in a plane. You have one heavy block of concrete which have two handles on the top for you to hold on to. Then someone pushes you out of the plane, while you’re holding on to the handles.
    Now, if we can assume you had the strength to hold on to the block as it was falling, the aim is to survive the fall by jumping off the block just before it hits the earth, so you only fall the short distance from the block to the ground.
    For this to work (in my head), you would need something that is heavier than your weight to push off from (hence the concrete block)

    So can you survive this? Can you illuminate the impact of such a height by using something like a concrete block to jump off from before it hits the earth?

    And for the truly crazy, you could have a small ejector seat type device on the souls of your trainers (special trainers that is) that activates the moment you let go of the handles connected to the block of the concrete - so you would launch into the sky where you could then activate your parachute to float gracefully down to earth :D]

    Anyway, after you've stopped laughing at my pathetic attempt to interpret the laws of physics, could you explain it to me? (in the simplest way possible please)

  2. jcsd
  3. Jul 15, 2003 #2
    Doesn't work that way. You'd still be going WAY too fast downward even while jumping off the block. All contestants would go home a blood stained concrete block.

    Edit: You'd be ok with an ejection seat if it threw you far enough up for the chute to open correctly.
    Last edited by a moderator: Jul 15, 2003
  4. Jul 15, 2003 #3
    That depends on how long the springs of your ejector seat are. Because it's acceleration (not velocity) which is dangerous.
  5. Jul 15, 2003 #4
    im thinking comical springs like in tom and jerry, but reality speaking, you would need to fire high into the sky, so acceleration has to be pretty quick if your just about to hit the ground.
    maybe a rocket pack....

    Man im thinking about it so much i want to try it out just to see :)

    So it wouldnt work?...damn.

    Fun though, fall out the sky like a commet, then blast back up, then flote back down hehe, thanks anyway
  6. Jul 15, 2003 #5
    I understand from your original post that u somehow think that jumping from the concrete block just before it hits the ground would be just like jumping from a floor with the same hright.
    Well, if you are doing so, you are kind of wrong, cause you have the same velocity as the block that u are on (just before jumping).
    Now, what will really affect if you will survive or not (assuming you are somekind of geosphere iow, forgetting the position in which you are falling) is your speed the moment you hit the ground.
    Now, as i said before, just before jumping from the block, your velocity is the same as the block's velocity (hence your speeds are equal), the moment you start jumping, you are affecting the block with a force, and the block will affect you with an equal (but opposite in direction) force, therefore the block (As u jump) will add some momentum upwards to you.
    Now, if the momentum the 'jump' adds to you upwards is equal to the original momentum (downwards) that you have, you will be free falling (just like if you let yourself fall from a building with the same height as the block's height).
    If the momentum of the jump is bigger than the original momentum, it would be like if you are jumping (with a velocity upwards) from a building with the same height of the block, or free falling from a building higher than the block when u jump.
    If the 'jump' momentum is smaller than the original momentum, it will be like if you are diving from a building with the same height, or free falling from a heigher building (except for time needed).
    Let's analyze it mathematically.
    Say, the speed of the block when you jump is v1, its height if h, the force that acts on you upward is f, and it acts for t time, your mass is m.
    (1 : Just before you jump
    2 : just after you jump
    3 : when u reach the ground)
    Mom(entum)1 = mv1
    ft = [del]mv = m[del]v = mv2 - mv1
    mv2 = ft + mv1
    Let's define up as positive, down as negative, then :
    |mv2| = |mv1| - |ft|
    (now we are dealing with scalars, not vectors)
    v2 = v1 - ft/m
    The potential energy when you jump (comparing to the earth's surface)
    PE2 = mgh
    E2 = E3
    PE2 + KE2 = KE3 + PE3
    But PE3 = 0 (when you hit the ground you don't have potential energy to the ground, since the distance between you and it = 0)
    PE2 + KE2 = KE3
    mgh + 0.5mv22 = 0.5mv32
    v3 = Sqrt((mgh + 0.5mv22)/(0.5m))
    = sqrt(2gh + v22)
    but : v2 = v1 - ft/m
    v3 = Sqrt(2gh + (v1 - ft/m)2)
    Or ... if you want f=ma --> f/m = a
    v3 = Sqrt(2gh + (v1 - at)2)

    Is this usefull ?
  7. Jul 15, 2003 #6


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    And people said I was crazy for skiing behind a dirt bike on a mountain board.
  8. Feb 4, 2004 #7
    "if we can assume you had the strength to hold on to the block as it was falling"

    all things fall at the same speed. a giant 200 tonn block falls at the same speed as a 1 tonn block. so you would not require any strenth to hold onto the block, apart from the closing hand action.

    but, jumping off the block at the last second is entirely acceptable, *as long as you can jump fast enough*

    and by fast enough, when a normal person jumps, they get about 5 mph. maybe 10. if you are falling at 300 mph, and jump up, you're now only falling at 290 mph. so in order to survive you'd have to be able to jump up at over 300 mph. you know, leaping over tall buildings and stuff. plus with the fact half your force will be going to push the block down, you're going to need to be able to jump up at near mach 1. which reminds me of this cartoon called The Maxx. in this cartoon world was this race of small creatures, that had the ability to jump up at near mach one. but had no way of surviving the landing as they come back down. luckly you have that parashute, huh?


    v being the speed you're falling, times 2, ='s the speed you need to be able to jump up to live.
    Last edited: Feb 4, 2004
  9. Feb 8, 2004 #8
    yOU WOULD HAVE rto jupmmp at at veruy afaswt rate, yo umight hav etop ototo figuyer 08yn yusy tsy tsyr rayte fors5t foisrt adm tjewm tru it bi t basicxally I'll know it when I se it..

    ...wow, I had a few too many, what I mean is that we've all heard of super human feats in which under extreme stress occasionally people tap into adrenaline or all sorts of unknown reflexive or thinking capacities and so it's possible, but if you could do it you would first have to understand that you could do it before you did it, I'm not saying people can fly if they set their mind to it, I'm saying that people can fly if they find a way and they have found many ways.

    ...the important thing to remember is that even superman puts his pants on one leg at a time.
    Last edited by a moderator: Feb 14, 2004
  10. Jun 18, 2004 #9
    Ok, so you would need to jump upwards at a speed at least equal to the speed of the block.
    Anyone happen to know the terminal velocity of a block of concrete?
  11. Jun 28, 2004 #10
    i always think that, like if i was in a building that was colapsing, i could have something below me and just as it hits the ground I could jump and survive, but then i thought about it. You would be accelrating so fast until you reach ur maximum velocity (if you have not hit thr gound yet) and if you jumped you would only stop yourself from falling so fast, i.e. you were fallin at 200mph downwards, you jump you would land at around 150mph (this is ONLY a guess!),

  12. Oct 25, 2004 #11
    It doesnt quite work that way, when in a free fall given excellent conditions such as no friction or wind resistance you the person and the object in this situation have the same velocity. The velocity of gravity is 9.8 m/s^2. When you get close to the ground you are going at a great speed. By pushing off of the block you would then increase the blocks velocity by providing a force to it. Your velocity would not change much if at all. Imagine doing this off of the top of your house, now imagine jumping out of a plane, you would break your neck or kill yourself jumping off of the house, and you would deffinately kill yourself if you jumped out of a plane holding a concrete block. Try it and let me know how it works.
  13. Feb 1, 2005 #12
    Let's examine three cases:

    1: You jump off the block right after you both leave the plane. The block gets a kick down, you get a kick up, and hover slightly, then fall under gravity. You fall all the way to the ground, and splat.

    2: You jump off the block right before you and the block hit the ground. The block hits the ground going a little bit faster, then you hit the ground going a little bit slower, but still _REALLY_ fast, so...splat.

    3: You jump off the block with superhuman strength right before you and block the ground. The impulse you must give yourself to stop yourself, is how much...well, exactly the impulse that the _ground_ gives you to stop you when you hit in #1 and #2 above. Since this was enought to make you kill you then, it is certainly enough to kill you now. The picture is that you jump _so hard_ that you do as much damage to your self as the ground will do.

    So, bottom line is, after you have acquired that much energy, there is only one way to land safely -- you must decelerate gradually, over a long distance, so that the impulse is the same (enough to stop you), but the instantaneous force on your body is bearable (i.e., less than will rip you apart). Falling in a giant column of shaving cream might do it. Same impulse, longer stopping distance, less force, you live.

  14. Feb 1, 2005 #13
    Watch the myth busters, they do this very experiment, poor buster, he was TOOOOOAST LOL. Its on the discovery channel. They did it in a falling elevator with a spring, exactly as you are inquiring.
  15. Mar 31, 2005 #14
    I believe this is not possible to jump off the the block to slow yourself down because even if you are holding on to them before you jump off, you will eventually have to release before you push up, otherwise you wouldn't achieve your maximum push upwards. Nevertheless, pushing on air...that's kinda useless, I don't think air resistance would provide enough force for you to push against to obtain a slower speed. Its sorta like swimming I guess, if you have an object in the water, and its in free fall, lets say its like floating or something. When you push off the object it doesn't really help you move in the other direction, although this case, there's alot more resistance in water than in air so you do move a bit. I may not know how to prove my explaination by mathematics, but I sure do know this is probably one of the most logical answers.
  16. Mar 31, 2005 #15
    During some point, you will reach terminal velocity. This is about 150mph. Converting to meters per second gives about 66m/s. Lets say the person has a mass 60kg and the block is 20kg, total mass 80kg.

    Your kinetic energy is given by .5mv^2, equivalent to 174240 joules. You would need to dissipate 174240 joules on impact to the ground.

    Energy dissipated by a spring with spring constant k,

    [tex] \int F dx = \int kx dx = kx^2/2 [/tex]

    An industrial spring has a spring cosntant of about 10^5 so:

    [tex] 174240 = (10^5)(x^2)/2 [/tex], displacement of 1.86m.

    Now, [tex] W = F d [/tex] 174240 / 1.86 = 93677N, or divided by 80kg, an average acceleration of 1170m/s^2, or 120g's.

    Thats more g's than you'd ever want to go through.

    If you wanted to go through maybe 10g's, which is much more reasonable,
    working backwards:

    [tex] 10 g = 98m/s^2 = 7840N [/tex]

    [tex] W = Fd = 174240 = 7840d, d = 22m [/tex]

    A 22meter spring would do the job about right. You would need a spring with spring constant k = 356 N/m, which isn't too much.
    Last edited: Mar 31, 2005
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