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

Glass with water

  1. Jan 31, 2007 #1
    1. The problem statement, all variables and given/known data
    What is the velocity of the water at the top of a glass of water? Is it really 0?
    For example, there is a lot of problems which asks with what velocity does the water would leave that glass if I make a hole on the bottom. For these, we consider the velocity at the top to be zero. Why is that? Is it approximately zero or really zero? I am thinking now, if it is an incompressible fluid, it must be zero, because if the water is confined to that volume, cannot have speed. Nevertheless, it is clear that the water molecules are randomly moving. Can you clarify my doubts? Thanks in advance.
  2. jcsd
  3. Jan 31, 2007 #2


    User Avatar

    Staff: Mentor

    When there is no hole in the bottom of the glass, the velocity of all of the water is zero. When there is a hole suddenly made in the bottom of the glass that goes all the way across (hole diameter = glass diameter), all the water accelerates together down out of the glass, and the velocity of the top surface is the same as the velocity of the bottom surface (ignoring the wetting effects on the walls).

    When there is a hole in the bottom that is smaller than the diameter, then the flow rate out the hole will determine how fast the top surface goes down (through volume change calculations).
  4. Jan 31, 2007 #3
    I had an exercise on my book where I should show that the velocity at the bottom of the glass where there is a small hole is [tex]\sqrt{2gh}[/tex], h is the height of the water level. This is only true, if I consider the velocity at the surface to be zero, when applying Bernoulli's equation. Right?
  5. Jan 31, 2007 #4
    which it will nearly be if the hole is relatively small.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook