# Fluid in a syringe

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How would you go about solving the following problems regarding a syringe full of water?
First, find the magnitude of force required to be applied to a piston of an 85ml syringe with a 60mm diameter tube to drain the tube in 25 seconds through an outlet of 10mm diameter?
Second, what would the pressure of water be during this process in both the syringe and in the 10mm outlet?

Any help would be appreciated:)

anorlunda
Staff Emeritus
I moved this thread because it sounds life homework.

@flutrpooch , you must show your attempt at the solution before the helpers can help.

Chestermiller
Mentor
Is there any needle attached to the syringe? What is the length of the 10 mm outlet?

Delta2
Homework Helper
Gold Member
What equations can you setup for this problem?
I suggest you setup
1) one equation from the continuity equation that will relate the two cross section areas of 60mm and 10mm diameters and the respective velocities.
2) one equation from the fact that the fluid will be all drained within 25seconds
3) one equation that the rate of work from the force (plus possibly the work from the weight of the fluid, depends how the tube is oriented, vertical or horizontal) equals the rate of change of kinetic energy of the fluid in the tube and of the fluid in the outlet

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There is no needle involved and the 10mm outlet could be given a length of 10mm also.

haruspex
Homework Helper
Gold Member
There are typically two elementary level types of question re incompressible fluid flow.
1. Work conserving, i.e. no drag. Use Bernoulli's equation (equivalent to part 3 of Delta2's reply).
2. Laminar flow with drag.
I would guess you are supposed to be treating it as type 1.
Asking for the pressure in the narrow section makes no sense without further info. On what you have provided, you can only figure out the pressure difference between the sections.

Chestermiller
Mentor
Are you sure of the dimensions of that syringe: 6 cm diameter, 3 cm long? I’m thinking more like 2.5 cm diameter, 17.3 cm long.

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• Delta2
No , syringe dimension is ok

Delta2
Homework Helper
Gold Member
Which of the cases 1. or 2. of post #6 are we dealing with? Is there friction between the walls of the syringe and the water?

Case 1 and we can ignore friction between walls and the water.

haruspex
Homework Helper
Gold Member
Case 1 and we can ignore friction between walls and the water.
Then follow the steps Delta2 set out for you in post #4.

• Delta2
Delta2
Homework Helper
Gold Member
You haven't show us any attempt, I see you are new to Physics Forums (PF). According to the rules of PF you have to show us your work attempt with as much detail as you can before we ll be able to help.

You can write a decent post with equations and lots of math symbols even if you dont know ##LATEX## commands if at toolbar of the reply edit box, you press at the capital sigma icon ##\Sigma## and you ll get all sorts of symbols you can click to and are useful for math equations.
for example

∫S1v1dt=°S2v2dt, ϑv1/ϑt=0

are some equations I wrote using those symbols.