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Pressure on block immersed in water?

  1. Sep 18, 2011 #1
    i was solving a problem based on archimedes principle. but got confused whilr determinig the pressure.
    well the situation was that a block is immersed in water. the height of water column above it is .05m. the blocks side is .02 m and its base area is 25cm^2.and the height of water below the block is not given.
    the presure on top of the block was simply the application of formula 'pgh' = 500pascal.
    but how do i determine the pressure on the lower surface ?

    also in general while determinig pressure on lower surface i THINK that we consider the height of water column from the upper edge of block to the bottom of the water container.
    if this is true then why?

    Attached Files:

  2. jcsd
  3. Sep 18, 2011 #2
  4. Sep 18, 2011 #3
  5. Sep 18, 2011 #4
    From your picture it looks to be .07m
  6. Sep 19, 2011 #5
    yes the question was solved in my book using .07m
    but why? i mean we have to find the pressure acting on the lower surface. so why not consider the height of water under it?
    i dont understand this point.
  7. Sep 19, 2011 #6


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    Staff: Mentor

    It's the weight of the water above (and the weight of the atmosphere above that!) that is is creating the pressure at any given depth. The water below only provides a "surface" upon which the weight acts to manifest the pressure. If this were not the case the oceans would leap from their beds, there being no water above them to hold them down against the pressure of all that water below the surface level :smile:
  8. Sep 20, 2011 #7
    ok...you have a point
    but then according to what you are saying , the pressure on the lower surface should also be in the lower direction and thus there will be no upthrust. pressure on both surfaces will be directed downwards.....
    also i remeber when i was in 8th grade there was a problem similar to this one and the book authors while computing pressure on lower surface considered the water column to be = height of block+water under it
    so i am all confused...explain a bit more
  9. Sep 20, 2011 #8


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    Staff: Mentor

    Pressure is caused by the weight of the material above, but it isn't weight itself. Gases and liquids are composed of highly mobile particles that distribute the force in all directions. The weight of the material above sets the magnitude of the distributed force at any given depth.

    Pressure on an immersed object is always directed inwards, normal to the surface of the object. Upward facing, downward facing, sideways, or slanted, it makes no difference.

    The problem you're remembering must have used a coordinate system that had its origin at the bottom of the container, and likely determined the depth as the difference between a given height in the column and the height of the surface.
  10. Sep 20, 2011 #9
    ok ...thanks
    i never really knew that pressure was always directed normally inwards
    now i understand that pressure in other words is a distributive force due to weight of material above it
    so by above understanding the pressure on lower surface is also directed inwards or in other words:it is directed uipwards and its magnitude is more than at the top surface
  11. Sep 20, 2011 #10


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    Science Advisor

    It's just the way fluids work (and hence don't hold their shape) -- they try to distribute forces in every direction they can, on any surface available.
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