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Help with a question

  1. Aug 5, 2004 #1
    pls help me with this question:

    Why can a person run on thin ice (over a pond), but cannot stand on it?
     
  2. jcsd
  3. Aug 5, 2004 #2
    Think of it this way.

    When one stands still, all of his or her weight is a slave to gravity, forcing pressure perpendicular to the ground.

    But when this person runs, part of their weight is moving forward as a result of momentum (and all that good stuff). It's almost as if, according to the ice, the runner weighs less.
     
  4. Aug 6, 2004 #3
    that answered it....but i can stand perfectly fine on ice, mate....
     
  5. Aug 6, 2004 #4
    I don't think it is true that you can run on thin ice on which you can't stand. It is also not true that the gravitational force is smaller when you run; the pressure you excert is actually increased by you pushing off the surface and landing again.

    What does happen when you run is that you have direct contact with the same piece of ice only a small amount of time. This might not be enough to break the ice and allow water to flow out of the gap, and you fall in.
     
  6. Aug 6, 2004 #5
    It's also important to remember that you can break ice while running. I'm sure many of the competitors on "World's Strongest Man" would be very likely to break ice even while running on it. :biggrin:
     
  7. Aug 7, 2004 #6

    HallsofIvy

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    That's completely wrong. Force is a vector quantity and its horizontal and vertical components are independent. weight is always vertical (in a sense that's the definition of "vertical"!).

    Running on ice does NOT apply less force to the ice at any one point (in fact it applies more) but it applies it for a shorter time
     
  8. Aug 7, 2004 #7
    Okay, so maybe I worded it wrong. Forgive me.

    Christ.

    edit: Sorry, that was kind of a jerk thing to say.
     
    Last edited: Aug 10, 2004
  9. Aug 14, 2004 #8
    Yeah, I would think that running on the ice would be the worst thing for breaking the ice, walking would come in second and sliding on the ice would come in third.
    You might stand on something and it doesn't break or it just cracks but if you STOMP on it (as in running), it's definitely gonna' break.

    Perhaps there's an advantage to running in that you might get out of the foot space fast enough (before the ice breaks and you fall through) but sliding quickly across the ice would be even safer because you can get out of the space just as quickly as running (plus, you aren't stomping like you are when running).

    I think most people would be tempted to tread softly and keep looking for thicker patches of ice, if possible. Best not to be out there in the first place!
    :smile:
     
  10. Aug 14, 2004 #9
    Reminds me of an episode of the tv show Mythbusters, in which they proved that running in rain gets you more wet than walking in rain does. Unfortunately I didn't get to see how they conducted the tests.
     
  11. Aug 15, 2004 #10

    HallsofIvy

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    Now I SAID that running applies the same force (one's weight) to he ice for a SHORTER time. It is quite possible that running across then ice might actually work: because you don't spend enough time at any one point for the ice to break.

    The only way I can think of to test that is to have people run across ice. Any voolunteers?
     
  12. Aug 16, 2004 #11

    LURCH

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    As a Michigan resident, I have conducted this experiment several times (all unnintentional). It works. In the same way skis will keep one up out of the water at speed, but let you sink if you slow down, ice that would break underfoot if you stood on it will hold you up long enough to get away if you run. I think this is where the cliche' about "skating on thin ice" became popular; while skating, it is possible to travel otu over a patch of ice that will not support yopur wieght without even knowing it. Then, when the skater slows down or stops, they find themselves conducting their own involuntary science experiment.

    Most of these unwilling researchers achieve breakthroughs!
     
  13. Aug 16, 2004 #12
    So when people rescuing someone after having broken the ice lie down, is it to distribute their weight over a larger area? I think I've seen ladders put on the ice used, too.
     
  14. Aug 16, 2004 #13
    Running: When running you put 2-3 times your body weight on the ice, and about half your foot (the ball of your foot) pushes off the ice to proplel you forward. Although the foot-to-ice contact time is small, ice is a crystal, and breaks very quickly if a large force is applied, regardless of contact time (that is to say, contact time is a less important variable than the weight variable).

    Walking: Walking puts about 1.5-2.5 times your body weight on the ice, and once again, about half of your foot pushes off the ice to propel you forward. Contact time is about 2x or 3x as long as when running, but this excess in contact time would not significantly increase the chance of cracking the ice, since your weight is reduced, and weight is the more important variable when dealing with a crystal structure like ice.

    Sliding: Of course sliding is the best option... First off, your whole foot (both feet lets assume) are in constant contact with the ice and therefore your full body weight is distributed over both of your feet as you slide on the ice (as opposed to half of one foot in running/walking). More importantly, you do not 'push' off the ice, so only your full body weight (1x) is used to crack the ice.


    The vector guy: I thought your example was smart and correct... You said that when you stand still, gravity is the only force acting on you and it acts downwards (perpendicular to the ice surface), thus exerting great pressure. But when you move forward, a new force component acts parallel to the ice surface, and thus spreads out the load of gravity pushing you downwards. The guy who tried to debunk you was a freshman, I can tell.
     
  15. Aug 17, 2004 #14

    LURCH

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    Has any of you seen that Asian guy that runs on a long path made of floating reeds? Pretty amazing!
     
  16. Aug 17, 2004 #15
    Sounds cool. Have a link to it?

    On a similar note, have you ever heard of that gecko/lizard type thing that can run on water?
     
  17. Aug 18, 2004 #16

    LURCH

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    Yeah, my sister had one of those for a time. Pretty cool.

    I went looking but could not find a link to the guy running on the floating grass mat, but I did find this link to the Basalisk.
     
  18. Aug 18, 2004 #17
    Unfortunately we're getting way off topic now. But it's worth it. Watch this video of a Basilisk Lizard running on water.
     
  19. Aug 18, 2004 #18
    That's not off topic, it's all about walking on water, isn't it? :smile:

    False vacuum, that mythbusters episode sounds a bit sketchy to me.

    Been busy but finally thought about this question a little more.
    One example which may show that running could cause a footfall to land harder than walking is that joggers and runners have more knee problems than walkers do, even if they run relatively short distances.
    Running/basketball shoes have more shock-absorbing support (or are at least are purported to have) than regular tennis shoes.
    This may have to do, in a way, with weight dispersion (or force distribution?) because at the point in time when the left or the right foot lands (while running), more of the weight (or force) is concentrated in that (left or right) leg and foot than in the other leg (and than it would be if walking).

    Additionally, I suppose the possibility of some kind of oddball reaction occuring in the ice can be factored in (as in Dr. Kaku's description of a diamond breaking when its nodal point is hit) when running as opposed to walking. In other words, the object being struck may have a disproportionate reaction.
    So, in this particular case, you are not just looking at the amount of force, you may have to also look at inestimable reactions (the degree of which may be unpredictable) in the object being struck.

    On the other hand, how hard a person hits something/how badly it breaks (in this case, ice) might actually have more to do with their strength than with their weight.
    (Of course in this next example, I'm assuming the person is trying to hit the ice harder rather than trying not to break the ice):
    Weightlifter #1 weighs 175 pounds and can do 300-pound legpresses.
    Weightlifter #2 also weighs 175 pounds but can do 400-pound legpresses.
    Which person could kick a better hole through ice? I'd say #2.
    So, at least in this case, the person's weight has nothing to do with how fast and how hard of a hole he could make if he were trying.
    There is no doubt that length of time that the foot remains on the ice makes a difference but it's probably not the sole (for lurch) factor.
    It would seem that there are many considerations- some which may produce only slight variations in estimation and perhaps others that could produce considerable difference in outcome.

    I'm just throwing out ideas. Gadzooks, what a headache issues like this must be. Glad I didn't take engineering.
    Speaking of which, maybe a lurking engineering geek will be kind enough to state the textbook terms and outcomes (the predictable ones) for our sprinter and our walker.
    Or we could HallsofIvy's advice and tie a rope and a flotational device around Lurch.

    Hm. I shouldn't skim the posts, I missed reading a few of them.
    Okay, here's comments to several:

    (#2)False Vacuum,
    Yes, the second paragraph (as many have stated) is more a result of "getting the hell out of dodge" quicker. Before the ice has time to crack. Soggy feet, however, may be another issue. :-D

    (#4)da_willem,

    There are a lot of factors that determine whether the person falls through. The two main ones are, of course, how big of a hole his foot creates and, secondly, how strong the next patch of ice he lands on is.
    He may only come out of it with soggy feet (if the ice breaks real slowly) but he's also taking a big chance when he stomps on ice that the area that breaks will be too large for him to escape from.

    I think I would prefer the idea of laying down on the ice. Not just because of the weight dispersion factor that has been mentioned in another post but also, think of this:
    If there's the possibility that you're going to go under anyway, you might as well be laying down. For one thing, you won't fall as hard and as far underwater as you would have if you were standing up when you fell through and you will still have a better shot at maintaining your bearings so that you might be able to lift yourself up onto the (hopefully) stronger ice that surrounds the hole.

    The disadvantage to laying down is, of course, that you can't move around very efficiently. Unless you're being chased by a polar bear, though, I'd think it would be better to escape mother nature and Newton's laws slowly and patiently.

    (#5)false vacuum,
    Strength is another consideration. It would seem that someone with stronger legs than another person would run harder (land harder), whether or not their weight was the same. (sort of like a Holyfield punch as compared to a Jim Carrey punch)
    Unless the stronger-legged person were somehow able to be more careful and tread lightly (if possible). You would surely think that there could be some degree of control involved.

    (#11)Lurch,
    Your post remind me of Wiley Coyote. When he's suspended in mid-air, he doesn't fall until he looks down and realizes he's not standing on the cliff anymore.
    I guess the writer thought he'd use a little of the Shrodinger's Cat interpretation.

    (#12)false vacuum,
    The ladder might serve an additional purpose (other than just weight dispension) by anchoring it above one (or more) stronger patches of ice. The theory being that the stronger patches might still support the entire ladder even if part of the ladder is covering weak patches.
    I think I'd use a wooden one. Aluminum's lightweight but I know that wood might float.

    Haven't read Muon's post yet, though.
     
  20. Aug 18, 2004 #19
    lol falseVacuum,

    I've seen it in action before.

    I think its called a pondskipper.
     
  21. Sep 17, 2004 #20
    Hi,

    It may be that more force is delivered to the ice by the runner since his acceleration downward at the point of contact may be greater. (F=ma).

    Even if the acceleration is not greater, the velocity of impact may be greater thus transferring more momentum to the ice and in a shorter time. (F=dP/dt)

    juju
     
    Last edited: Sep 17, 2004
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