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Why does a fluid exert an upward force?

  1. Jul 28, 2009 #1
    I'm doing first year physics, and we've just stared a unit on Fluid Mechanics

    Our lecturer showed us a demonstration of suction, lifting up a table by attaching a rubber suction matt type device, with a handle on it. The point of the demonstration was to show that even though the rubber suction device didn't weigh much, the difference in air pressure between the normal atmosphere of the lecture hall and the air trapped beneath the suction cup was so great that considerably pore force had to be applied to break the suction.

    Now, my understand is, this is due to the fact that while the atmospheric pressure is exerting a downwards force on the suction object, due to the scarcity/low density of air trapped beneath it, that there is less upwards force being applied to the bottom of the suction cup, so the net force that one must overcome to lift it is greater.

    My question is, what is it about fluid that exerts a force upwards, or indeed, in any direction not aligned with gravity? The downwards acting force of the atmosphere I get, because it has mass and is under the force of gravity, but an upwards force exerted on an object I do not understand. What acceleration is happening at the surface of a fluid!?

    Sorry if I have explained this for me, let me know if you need clarification.
  2. jcsd
  3. Jul 28, 2009 #2
    The force/pressure exerts from every direction around the object. But the pressure get higher when you get deeper in the fluid (thanks to gravitation). So the net force is upward.

    Try using the suction cup under the water. You can get the net force in any direction exactly like when using it in the air.
  4. Jul 28, 2009 #3
    Yes, but my question is why?

    What inside a liquid exerts a force on its surroundings?
  5. Jul 28, 2009 #4


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    The atoms/molecules with their brownian motion hit everything around them and produce a net force so to speak. Surface tension acounts for a small part of preassure in every liquid and gravity plus depth presses on every layer beneath.

    If at 10 m in water you would have roughly one atm of surplus preassure then that means that the air above you on the ground has the same weight as the column of water. A column of atmospheric air of 1 sqr meter 50 km high would roughly weigh 10 tonnes.
  6. Jul 28, 2009 #5


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    Strictly speaking, in the interest of scientific accuracy, Brownian motion refers to the visible effect of molecular kinetic enrgy on suspended particulate matter in a fluid, not to the motion of the aoms or molecules themselves.
  7. Jul 28, 2009 #6


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    The force exists at a boundary because the total energy (technically, the total Gibbs free energy) of the system would be lower if the boundary moved a little in the direction of the force. (In fact, this is the origin of all forces. It all comes down to free energy minimization -- or equivalently, entropy maximization.)
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