Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculate difference in pressure?

  1. Oct 31, 2011 #1

    gibson101

    User Avatar

    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
  2. jcsd
    Phys.org - latest science and technology news stories on Phys.org
  3. Oct 31, 2011 #2

    gibson101

    User Avatar

    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

    User Avatar

    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

    User Avatar

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

    gneill

    User Avatar

    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

    User Avatar

    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

    User Avatar

    Staff: Mentor

    Yes, that looks much better!
     
    gneill, Oct 31, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook