# Determine the medicine's flow speed through the needle

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

## The Attempt at a Solution

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~christina~
Gold Member
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.
http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

height stays same so it cancels

solve for v

hmm i know that Av= Av and the equations but i keep gettin a wrong answer... A= 2.2x10^-5 m^2

~christina~
Gold Member
hmm i know that Av= Av and the equations but i keep gettin a wrong answer... A= 2.2x10^-5 m^2
$$P_1 + 1/2 \rho v^2 + \rho gy_1 = P_2 + 1/2\rho v^2 + \rho g y_2$$

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.

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well the pgys are 0 u have v= 2change in P/pv pv is A right? P=1.01 x10^5 and

~christina~
Gold Member
well the pgys are 0 u have v= 2change in P/pv pv is A right? P=1.01 x10^5 and

yes pgy is 0 so what is the equation then?

not sure what your saying when you say
v=2change in P/pv pv is A

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

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isnt that whats left of the equation?

~christina~
Gold Member
isnt that whats left of the equation?