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Force acting at an angle.

  1. Oct 22, 2012 #1
    "Force acting at an angle doesn't exert pressure on a surface". Is this statement correct or wrong?
  2. jcsd
  3. Oct 22, 2012 #2
    it is generaly wrong. The only angle wich wouldn;t make pressure is 90 degrees
  4. Oct 22, 2012 #3
    90 degrees with the surface?
  5. Oct 22, 2012 #4
    90 degrees as in parallel to the surface. Any force that has a component perpendicular to the surface will exert pressure on the surface proportional to that component. The other components of the force may not contribute to the pressure in general.
  6. Oct 22, 2012 #5
    What if the force is applied acting in a direction away from the surface?
  7. Oct 22, 2012 #6
    then it is a pull rather than a push...
  8. Oct 22, 2012 #7
    That wasn't the answer to my question, so let me rephrase it.

    If the force is a pull, does it exert pressure Sometimes?, Always?, Never?
    Last edited: Oct 22, 2012
  9. Oct 22, 2012 #8
    Well I am still ignorant of the situation you are refering to, but if the force is capable of pushing or pulling at the surface, then any component of the force perpendicular to the surface will contribute to pressure. Always. If you are refering to pressure of the atmospheric type, then there can be no "pulling", only pushing. So the angle would need to direct the force toward the surface.
  10. Oct 22, 2012 #9
    Let us suppose there are two identical blocks sitting on the table.

    So there is atmospheric pressure Pa acting on the top surface (area A) of each block.

    Now suppose we push down onto the top surface of one block with a force F, via say a push rod.

    then the total pressue on the top surface of the block is now

    Pa + F/A

    If we attach a sting to the top surface of the other block and pull up with a force F, that is insufficient to lift the block off the table,

    What is now the pressure on the top surface of this block?
  11. Oct 22, 2012 #10
    Pressure on the top surface would be equal to the pressure on the bottom surface. For the first block the Pa + F/A is the pressure required on the bottom surface to lift the block. For the block with the string it should be Pa - F/A.
  12. Oct 22, 2012 #11
    So what happens when F/A > Pa ?
  13. Oct 22, 2012 #12
    pressure will be zero and the resultant force will lift the block.
  14. Oct 22, 2012 #13
    OK got you. the force only partially contributes to pressure in that case.....The surplus does not :)
    Last edited: Oct 22, 2012
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