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Homework Help: Creek rocks

  1. Dec 6, 2008 #1
    1. The problem statement, all variables and given/known data
    If you have ever had to wade across a rocky creek while hiking in the mountains, you have probably noticed that by the time you get to the deep water in the center of the creek rocks don't seem to hurt your bare feet as much. what is the reason for this?
    a. one tend to stand on tiptoe in deep water, thereby reducing the area of the foot in contact with the rocks
    b. the greater pressure on one's feet in deep water means the rocks cannot dig in so much
    c. deeper water is colder, and hence more dense, than shallow water
    d. one experience a greater bouyant foce in deeper water
    e. the velocity of the water is less in deep region than in shallow regions

    2. Relevant equations

    3. The attempt at a solution
    I think the corect selection is either choice B or D. I am leaning towards B because I think the bouyant force is the same no matter how deep the water is.
  2. jcsd
  3. Dec 6, 2008 #2


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    Consider a force diagram at the point of a rock under your foot that you may be standing on.

    What determines the maximum pressure on your skin at the point of the rock touching your skin? Assuming the same rock and hence the same area of application why might the pressure at the point be less in deeper water?
  4. Dec 6, 2008 #3
    The force determines the decrease or increase in pressure assuming area is the same. So in deep water there must be something that lowers the force and ultimately the pressure felt by ones foot? would that something be bouyant force?
  5. Dec 6, 2008 #4


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    Isn't that what a bouyant force does?
  6. Dec 8, 2008 #5
    I thought bouyant force was more a function of the density of the liquid and the bouyant force was the same at all depths given uniform density of a liquid. Why does the bouyant force vary at different depths?
  7. Dec 8, 2008 #6


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    Buoyant force varies with displacement.

    Remember Archimedes in his tub?
  8. Dec 8, 2008 #7


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    As lowly_pion points out, if you were immersed up to your neck in the stream, bouyancy would be such that you would feel very little pressure on your feet. :wink:
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