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Is it less damaging to go faster?

  1. Nov 12, 2013 #1

    Simon Bridge

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    This thread on strategies for crossing a weak bridge reminded me of a passage in a kids book:

    Cannibal Adventure by Willard Price
    There's a scene where Hal and Roger Hunt are required to enter a village (as honored guests) by walking over the bodies of the village women. Hal takes a "when in Rome" attitude and walks normally, but Roger is concerned about hurting the ladies so he runs over their bodies "stepping lightly"... leaving aside the period ignorance (racism, sexism etc), would Roger's strategy actually work?

    I think the idea comes up a lot: going fast may apply more force but for less time - so there is net less damage. But I'm inclined to think, all else remaining equal, that going fast is a bad idea.
    OK - Roger shouldn't just stand on someones belly for like minutes at a time... but perhaps he should spend as little time as possible on just one foot - so his weight is distributed between two people?

    Could be one for the Mythbusters?
    The team could take turns walking over Adam?

    Note: If talking about using people like this is distressing - imagine these are weak boards on a suspension bridge or something. Or maybe these are adults trying to get children to safety by making a bridge of their bodies - how should they instruct the kids to walk?
     
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  3. Nov 12, 2013 #2

    phinds

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    Seems to me the problem is more complicated than your statement implies because the statement of the problem only considers the point of view of the runner, but shouldn't there also be consideration of the characteristics of the thing being stepped on? A weak bridge plank, for example, might be able to withstand very briefly a larger force than it can withstand for a longer period whereas some other weak bridge might be the exact opposite. A person might find a brief application of 200lbs either more or less distressing than a longer application of 150 lbs.
     
  4. Nov 12, 2013 #3

    berkeman

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    I think this should be applying the same force for less time. mgh isn't changed by moving quickly over the surface, and if you are at speed when you cross the initial threshold, there are no lateral acceleration forces applied to the surface...
     
  5. Nov 12, 2013 #4

    jedishrfu

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    How about the case of running on ice, if you move faster you could outrun the cracks because it takes time for the crack to crack.

    Another case occurs in Karate: If you punch someone and quickly retract your fist its less impact and thus less effective.

    Sometimes, people would do this to break a board and because of the retract the board flexes. In contrast, someone who punches to a point beyond the board will usually break given sufficient force.
     
  6. Nov 12, 2013 #5
    But when running/jumping you use a lot of force to 'launch' and also the impact from when your foot lands is much more serious, than when you put it down just walking.

    When running the amount of force for each step is greater but also the distance between steps is greater. To answer the question, I think that when you run on people it really hurts when your foot lands but if you walk carefully it's not so bad.
     
  7. Nov 12, 2013 #6

    berkeman

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    Valid points, but I envision increasing the leg cadence in proportion to the increased speed. That would hold the applied downforce per unit time constant...
     
  8. Nov 12, 2013 #7
    Yes, for the sake of working with the problem of which, walking or running, requires more force.

    When considering what hurts the people least, there's basically a shortcut ;)
     
  9. Nov 13, 2013 #8

    Simon Bridge

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    It's more interesting than it appears at first sight isn't it?
    A lot of apparently simple questions can be like that.

    The higher "launch" force would be taken on the ground ... Roger can have as long-a run-up as he likes.

    If he could jump clear over the walkway, then he could air-run over the bodies just touching each lightly with a toe... so lets say the path is longer than he can long-jump.

    Hmmm... Roger would need to do a flat run. A high bouncy run would be counter-indicated.
    Then you can keep the vertical change in momentum the same - would there still need to be a higher horizontal change in momentum - bearing in mind the losses in real life systems like running bodies?

    I suspect a lot will depend on the biomechanics of running vs walking.
    I wonder if anyone has studied how hard feet hit the ground at different speeds - you could use force transducers in shoes of someone on a treadmill perhaps?
     
  10. Nov 13, 2013 #9

    jedishrfu

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  11. Nov 13, 2013 #10

    Simon Bridge

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    I figured the shoe manufacturers would have studied something like this... well done jedishrfu :)
    Next - try the tests running on ballistics jell?
     
  12. Nov 13, 2013 #11
    Well this kinda reminds me of the thing I always think about when driving over railroad crossing.
    We can take this analogy Simon gave and compare it to say the stress that a car feels when driving over a gap in the road.

    I kinda think that if the gap is pretty small like those from rails crossing a road , then doing higher speed the wheel doesn't have the time to make a full amplitude from the position before the gap to the lowest point in the gap and back because it takes time for the shock absorber to change position , so from such a point of view a car get's less impact as it literally " flies" over the rail gaps.

    Now to take this into your walking over humans analogy here you don't have gaps but I would say that in order for each person to feel less force the runner would have to make smaller steps over a larger amount of bodies.When making a bigger step the force that you exert on the ground or body in this case is much bigger also the landing force when you close the step is bigger as your speed and the distance traveled also.
    So smaller steps and more steps in count over more bodies for a given distance I guess would do less harm.

    Well that's just my 2 cents.
     
  13. Nov 13, 2013 #12

    davenn

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    interesting thoughts, Simon

    It leads me to the other situation where there are several objects floating in water, say a pond or a stream. None of which on their own would support a persons weight, but if "hopping" quickly across them you would get to the other side without getting too wet

    cheers
    Dave
     
  14. Nov 14, 2013 #13

    Simon Bridge

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  15. Nov 14, 2013 #14
  16. Nov 14, 2013 #15
    If you (very roughly) obtain the impulse by taking the area under the curves of figure 6 in the above paper, it would tend to suggest the "fast walk" is a better option.

    Interesting one though Simon. Nice thread.
     
  17. Nov 14, 2013 #16

    Simon Bridge

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    It's looking like a general consensus on strategy isn't it?
    Walk briskly but don't run, and don't put the heal down first - probably try to plant the whole soul of the foot at once - get up speed while still on the ground, and minimize the up-down motion.

    Which suggests the first woman gets the worst of it as she's the step up - maybe a slight jump off the ground there.

    And we do not hop on Pop.
     
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