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Homework Help: Force and flow speed of a syringe

  1. Nov 27, 2007 #1
    1. The problem statement, all variables and given/known data

    A 1.8mL syringe has an inner diameter of 5.6mm, a needle inner diameter of .26mm, and a plunger pad diameter (where you place your finger) of 1.0 cm. A nurse uses the syringe to inject medicine into a patient whose blood pressure is 140/100.
    a.) What in the minimum force the nurse needs to apply to the syringe?
    b.) The nurse empties the syringe in 2.2s. What is the flow speed of the medicine through the needle?

    2. Relevant equations

    Q = delta V/ delta t
    Q = vA

    3. The attempt at a solution

    I have no idea how to start part a. For part b I tried the first equation to get a Q of 8.18X10-7. Then I used the second equation to get v = 3.852.
  2. jcsd
  3. Nov 27, 2007 #2

    Shooting Star

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    Homework Helper

    Actually, I don't understand the meaning of the symbols you are using. Would you elaborate a bit, and also the formulae you are using? Then perhaps I can help you.
  4. Feb 23, 2009 #3
    i think applied force is the function of frictional forces b-n piston and syrenge and frictional force due to flow.

    if u follw this finally you will end up with darcy weisbach formula.
  5. Feb 23, 2009 #4


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    This kind of depends on what level you're in. If this is a physics problem, then it appears to be a simple "hydraulics" problem. You have a small area of the needle, which is in a fluid of given pressure. In order to flow the fluid, the pressure inside needs to be greater than the blood pressure. Basically, what force on the plunger needs to be exerted to create enough pressure.

    Now, it appears as if this is the case. The previous post mentioned the Darcy-Weisbach formula, which you shouldn't need, for two reasons now that I think about it. First, in order to calculate pressure losses in a system like this, you would need the length of the needle, which you're not given. Also, head loss is a function of velocity squared. You can assume negligible velocity at which point there is essentially no pressure loss.

    So, you're left with a simple problem that is analagous to "find the force the man needs to exert to lift the car"
  6. Feb 23, 2009 #5
    I was supposed to have got a word in edgeways
    if I Understand the symbols in this forum correctly
    but my post has got deleted somehow.

    Stop mucking me around.

    I'm qualified and willing to help (MODS)

    Last edited: Feb 23, 2009
  7. Feb 23, 2009 #6


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    Gold Member


    [tex]F_{2}= \frac{A_{2}}{A_{1}}F_{1}[/tex]


    These are all you need to know to help you solve the problem
  8. Jun 12, 2009 #7
    I have this same problem. just different numbers, i just dont know where do start.....
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