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Homework Help: I NEED HELP tyvm

  1. Apr 8, 2008 #1
    A hypodermic syringe contains a medicine with the density of water (see figure below). The barrel of the syringe has a cross-sectional area of 2.20 10-5 m2. In the absence of a force on the plunger, the pressure everywhere is 1.00 atm. A force of magnitude 1.80 N is exerted on the plunger, making medicine squirt from the needle. Determine the medicine's flow speed through the needle. Assume the pressure in the needle remains equal to 1.00 atm and that the syringe is horizontal.

    second question

    An 11.3 kg block of metal is suspended from a scale and immersed in water, as in the figure below. The dimensions of the block are 12.0 cm 8.7 cm 8.7 cm. The 12.0 cm dimension is vertical, and the top of the block is 5.00 cm below the surface of the water

    i know that P=Po + pgh p=m/v and g i cant figure h out
    can u direct me a bit with this ty verymuch
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    Last edited: Apr 8, 2008
  2. jcsd
  3. Apr 8, 2008 #2


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    height stays same so it cancels

    solve for v
  4. Apr 8, 2008 #3
    hmm i know that Av= Av and the equations but i keep gettin a wrong answer... A= 2.2x10^-5 m^2
  5. Apr 8, 2008 #4


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    [tex] P_1 + 1/2 \rho v^2 + \rho gy_1 = P_2 + 1/2\rho v^2 + \rho g y_2 [/tex]

    don't forget atmospheric pressure is at the end of the needle solve for v2

    P=F/A to find the pressure exerted over the area of the plunger (force exerted on plunger )

    After finding the equivalent pressure that is due to the plunger, add that to atm pressure.

    show me what you did and I'll tell you what your doing incorrect.
    Last edited: Apr 8, 2008
  6. Apr 8, 2008 #5
    well the pgys are 0 u have v= 2change in P/pv pv is A right? P=1.01 x10^5 and
  7. Apr 8, 2008 #6


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    yes pgy is 0 so what is the equation then?

    not sure what your saying when you say

    and the area is small on the end of the syringe but did they give it to you?

    Look at the post above (last post by me) I added stuff.

    I have to go but I'll see what you did tommorow (post your work)

    A2/A1<<1 -> note
    Last edited: Apr 8, 2008
  8. Apr 8, 2008 #7
    isnt that whats left of the equation?
  9. Apr 8, 2008 #8


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    what are you talking about.

    No the A1V1=A2V2 is the continuity equation and that is substituted into the bernoulli equation. And then you can solve for v2 which you want.
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