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Homework Help: Conceptual question: Bernoulli's Principle Syringes!

  1. Feb 2, 2014 #1
    1. The problem statement, all variables and given/known data

    I'm interested in an explanation of the forces/principles involved in the following scenario: There are two syringes, level with one another, containing the same volume of water. The difference between the syringes is the gauge of the needle; one is of greater diameter than the other. One wishes to empty both syringes in the same amount of time.
    2. Relevant equations

    To empty the syringes in equal time, will one plunger require a greater force than the other?
    How will the magnitude of the pressure on the walls of the two syringes vary?
    How will the speed of the fluid exiting the syringe compare?
    (Sort of vague) What can we learn by applying Bernoulli's principle to the problem?


    Thanks you guys. I'm not very physics-minded and have been studying for the MCAT lately. I feel I should be able to understand any scenario in terms of the underlying principles. Not too realistic, but I'm especially interested in this one!

    3. The attempt at a solution
  2. jcsd
  3. Feb 2, 2014 #2
    Hi Bismuthman. Welcome to Physics Forums.

    This is not a problem that one would solve using Bernoulli. Because of the tiny diameter of the needle, the fluid flow in this system will be dominated by viscous pressure losses. The intertial pressure variations associated with Bernoulli will be negligible. Have you learned about viscous flow yet? If so, have you studied laminar viscous flow in a tube?

  4. Feb 3, 2014 #3
    My understanding of viscosity is about as thick as a flashcard: the internal friction of a fluid, the resistance of the fluid to motion. I am aware of the distinction between laminar and turbulent flow. What exactly do you mean by laminar viscous flow? Am I to understand that the viscosity of the fluid increases in the needle of either syringe (moreso in the smaller of the two)?
  5. Feb 3, 2014 #4
    Edit: Thanks for you answer!
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