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Homework Help: Force, pressure and acceleration

  1. Dec 7, 2011 #1
    Our high school physics course recently covered pressure. I invented the following scenario to illustrate something I don't understand about pressure.

    Say you have 2 thumbtacks/pins. They have the same mass. You press each pin to a wall with the same force, but only the one with the thin point moves and slides into a crack in the wall that it has created. So only that pin accelerated. F = m.a, so if F and m are the same for both pins, how come a isn't?
  2. jcsd
  3. Dec 7, 2011 #2
    well friend the reason is; the thin pin exerts a greater pressure, beacuse of it small base area as compared with the "thick" pin. Hence, it waz able 2 cause a wreckage through wall by ur applied force, which inturn producd d motion!
    Generally, the pressure created by a certain force "F" is inversly proportional to d area in contact.
  4. Dec 7, 2011 #3
    Normal force, the one that didn't move had a force on it from the wall whereas the one which did move wasn't actually pressing right up against the wall since it was sliding into a crack so the wall wasn't pushing back as much.
  5. Dec 7, 2011 #4
    For a = 0, the Sigma F equal zero, because the F you gave in was equal with the normal pressure, but when a = something not zero, the surface's shape is deformed, the normal pressure is no longer as great as before, and the backforce was friction of the nail's surface and deformed table surface, no longer normal force, efen if normal force still exist, the normal force can't be as great as before because of the vector is not 100 % to the opposite way,

    to explain why the table wrecked.. well.. I think it's correlated with mohr's circle of tension-compression diagram.. or maybe molecular deform modelling.. I don't know.. I'm still a newbie, sorry for the messed up language too ^^
  6. Dec 7, 2011 #5
    Thanks. So I suppose the higher the pressure on an object, the lower the normal force of the object?
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