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Homework Help: Calculate difference in pressure?

  1. Oct 31, 2011 #1

    gibson101

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    Calculate the difference in pressure between the top and bottom surfaces of a cube of size 10x10x10 cm at a depth of 20 cm in water. At 100 cm?

    I know that P=F/A . and in this case force is referring to buoyant force so F=ρgV. And the density of water is just 1000 kg/m^3. And the area of the cube is just 1000 m^3. But where does the depth of the water come into play?
     
    gibson101, Oct 31, 2011
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  3. Oct 31, 2011 #2

    gibson101

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    I did ρgh x A = buoyant force. So (1000)(9.8)(.2) x 100 = 19600. So Pressure equals 19600/100 since P=F/A so P= 1960. But i think this is wrong.
     
    gibson101, Oct 31, 2011
  4. Oct 31, 2011 #3

    gneill

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    Staff: Mentor

    Your equation is correct for pressure at a given depth. It is not the buoyant force (which will be the difference between the forces exerted by the pressures at the bottom and top surfaces of the cube).

    Pressure operates on a surface, not a volume. The volume of a 10x10x10 cube is 1000, but volume is not a surface area. Also, 10cm x 10cm x 10cm is not 1000m3 -- watch your units! cm are not m.
     
    gneill, Oct 31, 2011
  5. Oct 31, 2011 #4

    gibson101

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    but my answer is correct right? 1960 kg/m^2?
     
    gibson101, Oct 31, 2011
  6. Oct 31, 2011 #5

    gneill

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    Staff: Mentor

    No. You're asked to find the difference in pressure between the top and bottom surfaces. Pressure is measured in Pascals (Pa), which is equivalent to N/m2.

    Use your pressure at a given depth formula to find the two pressures and then their difference.
     
    gneill, Oct 31, 2011
  7. Oct 31, 2011 #6

    gibson101

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    okay i see. the difference in pressure for both of them is 980 N/m2. Correct?
     
    gibson101, Oct 31, 2011
  8. Oct 31, 2011 #7

    gneill

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    Staff: Mentor

    Yes, that looks much better!
     
    gneill, Oct 31, 2011
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