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Syringe find min force

  1. Jun 10, 2009 #1
    A syringe with a plunger of diameter 2 cm is attached to a hypodermic needle with a diameter of 2.1 mm. What minimum force (in N) must be applied to the plunger to inject into a vein where the pressure is 16 torr above atmospheric pressure.

    P=F/A since P will be constant throughout F/Aplunger= F/A vein

    First 760torr=1 atm=1.01*10^5N/m^2, which gave me the pressure in the vein =2126.3. Then I said that P, which is 2126.3* area of the plunger (pi*r^2)
    2126.3*pi*(.002)^2=.00668 which sound be by answer- the F. Although this is not correct. Where am I going wrong?
  2. jcsd
  3. Jun 10, 2009 #2


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    Check too to see that you are using radius and not diameters.

    Diameter of 2 cm means radius or 1 cm = .01 m.
  4. Jun 11, 2009 #3
    Thanks- that was why I was incorrect.
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