Maximum speed of falling objects?

Discussion in 'Physics & Math' started by Dinosaur, Mar 2, 2004.

  1. Repo Man Valued Senior Member

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    http://www.straightdope.com/classics/a1_225.html

    More... http://www.straightdope.com/classics/a950414b.html
     
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  3. JesseLeigh Registered Member

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    97
    Falling/jumping into water

    Evening!

    I'm new here, but have been thoroughly enjoying this thread. Great forum, BTW.

    Re: falling/jumping into water, I find myself perplexed by the contradictory info on the net.

    Forty-one years ago, as a child, I jumped from a ninety foot high diving board (in Aberdeen, Scotland) into (IIRC) twenty-seven feet of water. Some people dove off the board but I wasn't quite brave enough for that. I raised my arms straight in the air above my head, and after jumping, I pointed my toes to make myself as aerodynamic an object as possible. I hurt the bottoms of my feet when I landed on the bottom of the pool, and my lungs were bursting (I forgot to take a breath just before entering the water), but other than that it was certainly a survivable experience. The water was not 'Concrete' as the movie The Guardian said it would be.

    I was only eleven at the time but I remember the incident well. I haven't seen the facility for decades so I don't know if it still exists, but my guess would be that it does.

    If all the science I've read on the net (and seen in that otherwise terrific movie) is remotely true, I should be dead. I'm not, and can't scientifically find an explanation for that - not to mention the liability issues that would present due to the mere existence of the pool in Aberdeen. :shrug: Any thoughts?

    Shalom aleikhem - Jesse.
     
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  5. Cannon Registered Senior Member

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    207
    So that is in-fact is correct would the constance acceleration of 2g's be twice the speed, or is there a compounding factor involved?
     
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  7. Dinosaur Rational Skeptic Valued Senior Member

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    JesseLeigh: Water would act much like concrete if you fell from 90-100 feet Belly Flop style. Water would not act like concrete if your entry is like a diver doing a Swan dive or using the entry you describe with toes pointed.

    Falling from 90-100 feet onto concrete would likely be fatal no matter how you were oriented. Falling onto a lawn, you might be better off with a spread eagle landing. I suspect that landing face up is better than face down. This might be better than landing feet first & trying to use leg muscles to decelerate more slowly.
     
  8. Sci-guy Registered Member

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    In an ideal enviornment there is no maximum velocity of an object because on earth the gravitational acceloration is 9.81 m/s2, but when accounting for the air resistance that an object would encounter you must observe the object geometric shape. It all depends on how areodynamic the object is.

    I heard the same sorty on television of a bullet after being fired in the air that landed and injured someone. They tested these accounts on mythbusters, you might want to watch the episode, it was a good one! Well not really because they all are good episodes.
     
  9. Tom13054 Registered Member

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    I am a retired Police Officer when we went to the range to shoot there was a test area behind the hill of another shooting range. Now I know this is only a ricochet so the bullet did not go as high up but on occasion we would get hit with a falling bullet it didn't kill but it sure hurt a lot. The speed of the ammo used was 950 ft a second at the muzzle.

     
  10. three-brane Registered Senior Member

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    I would think the density of the object would have alot to do with it. i am thinking of how the mass of an object curves space. your mass curves space. not nearly as much as the earths mass. and the suns mass is even more so, right. your thinking "no shit".
    well the mass of a collapsed star where it warps space to the point of tearing it. not even light escapes and it is traveling at the speed of light.
    the mass of a photon.
    my point is your falling into the curve of the space around the earth. if you were the mass of a star and the size of you, your density would be so great the earth would be actually falling into you. yet you could argue you fell to the earth.
    so i say yes density would matter.
     
  11. freziggity Registered Member

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    The max falling speed of anything is directly relative to the mass/density of the objects and the gravitational field of the test environment. On earth, gravity accelerates an object 9.8meters per second per second. On jupiter it is 9.8 per second per second times 2.5 Since Jupiter has 2.5 times earth's gravity. Other than that, friction will diminish the rate of fall for any object, but in a vacuum, everything falls at the same rate of speed. Cool question.
     
  12. Pinwheel Banned Banned

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    You can try chucking darts off the top of a skyscraper.
     
  13. Syzygys As a mother, I am telling you Valued Senior Member

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    Alright people, just a few observations and facts:

    1. It is a well documented FACT that bullets fired into the air can kill people. Happens quite often actually. To reach the highest speed at arrival, it is better to fire the gun at an angle, so the bullet would keep its spin, instead of tumbling down.

    2. There should be a dropping height limit, somewhere where the air starts to get thin. Those record parachute jumps shouldn't count because they started in the thin air area, thus they had the advantage of speeding up before reaching normal air. Thus it is not really a drop.

    3. Shape is everything in this matter, and bullets tumble coming down. Now we could talk about the penny coming down on its edge if we make it spin, before the drop. My guess is that the fastest shape is the raindrop shape, a spherical shape with a tail. That has the lowest natural air resistance ratio...
     
    Last edited: Jun 6, 2010
  14. Syzygys As a mother, I am telling you Valued Senior Member

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    This is incredible silly missconception. Because of the airdrag, shape is everything in falling....

    You are saying that a rocket-shape and a parachute would fall with the same speed assuming everything else (weight and cross-section) being the same...
     
    Last edited: Jun 6, 2010
  15. iceaura Valued Senior Member

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    There is, of an object accelerating only by gravity toward the earth.

    The pull of the gravity gets smaller as the distance from the ground gets larger, and decreases faster than the distance increases - so eventually, falling from much higher up doesn't add much to the final speed.

    Arrows fall faster than raindrops - and I doubt raindrops have a tail when they are falling.
     
  16. D H Some other guy Valued Senior Member

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    True, but completely irrelevant to the problem at hand. The Schwarzschild radius of a feather is about 10[sup]-30[/sup] meters; for a Nimitz class aircraft carrier the Schwarzschild radius is about 10[sup]-19[/sup] meters. General relativity is not relevant to this discussion.

    An object's density, shape and even surface texture are important because the object is falling through the atmosphere. Atmospheric drag builds with velocity. At low Reynolds numbers drag is proportional to velocity. Once turbulent flow kicks in the drag force grows with the square of velocity. As a falling object gains speed the drag force will grow to be equal to gravitational force. This velocity where drag force is equal to gravitational force is called the object's terminal velocity stops accelerating at this point, called terminal velocity.
     
  17. Syzygys As a mother, I am telling you Valued Senior Member

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    12,671
    Have you actually tried it?

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    What's your point by the way?

    1. Arrows are NOT natural.
    2. There is more to it, like density and cross-section.

    Anyhow you are just proving that shape does count...
     
  18. three-brane Registered Senior Member

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    but if there was two objects of similar size and shape, the difference one has twice the density, they would still have the same terminal velocity?
     
  19. Syzygys As a mother, I am telling you Valued Senior Member

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    Nope. A bowling ball has a bigger TV then a same sized ball made of styrofoam.

    "Terminal velocity varies directly with the ratio of weight to drag. More drag means a lower terminal velocity, while increased weight means a higher terminal velocity."
     
  20. D H Some other guy Valued Senior Member

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    Of course not. That has nothing to do with relativity, however (the point you were trying to make in post #27). It has everything to do with atmospheric drag.
     
  21. iceaura Valued Senior Member

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    yes.

    Arrows are less dense than water, should fall more slowly on that criterion alone. If shot straight up, watched carefully straight down, they will overtake even fairly large raindrops after just a couple of hundred feet of free fall, at most. Mist and the like just floats.

    Of course shape matters. But falling things, even liquids, do not automatically assume the best shape or orientation for maximum speed. Leaves from trees don't, for example. Air is complicated stuff to push through.
     
  22. D H Some other guy Valued Senior Member

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    That is a misconception. From http://www.newton.dep.anl.gov/askasci/gen01/gen01429.htm
    If the drop is larger like a raindrop in free-fall, it has a domed top and a semi-flattened bottom because as it falls it must push the air out of its way. That "upward" push of the air being displaced causes the falling drop to have a rather flattened bottom.

    Contrary to popular misconception, a free-falling raindrop is not shaped like a teardrop -- round on the bottom and pointy on top.​


    Using the equivalence relationship "1 picture=1000 words," here are 3091 words on the shape of a raindrop.

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  23. Syzygys As a mother, I am telling you Valued Senior Member

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    >a free-falling raindrop is not shaped like a teardrop

    I guess I misspoke. One of the best aerodynamic is the teardrop, at least that is what carmakers are trying to copy when they want to make a small airresistence car...

    I guess the rain is too soft and it bends, thus kind of parachuting itself. But making it from hard material, a teardrop would come down pretty much with the fastest speed possible...

    See the shape on the top:

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    Last edited: Jun 7, 2010

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