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1. Hi, I am doing physics coursework on finding viscosity of fluids by dropping a marble into fluids, finding terminal velocity, then using stoke's law to find viscosity. (using density of fluid, sphere, sphere diameter etc). I have completed all the practical, now just the write up
However ... I could really do with someone's help one the formula I would use, has anybody done this before for their coursework? I know some people will say 'just google it,' but I have spent far too long doing so, and right now have dozens of tabs and each formula is a slight variation on the other. I honestly have no idea which is the best/right one (my physics teacher is ill till after the christmas holidays so I can't ask him) ... they all put the viscosity in a different units, some do not give the required unit of density for the calculation (kg/g per cm/m/mm), some have weird units I have never heard of, some use a terminal velocity, some use variable velocity, some involve Brenner’s Law due to the influence of the boundaries of the tube on the sphere, and some from a few university lecture notes go into further complexities!!
Is there a simple formula that my measurements can easily 'slot' in to that someone can recommend, which will give me results which are not necessarily 100% spot on, but just a simple measure of viscosity that is near the value.
Thanks!!!!!
2. The formula is

Where
F is the viscous drag in newtons acting on a sphere of radius r, moving with velocity v (m/s) in a medium of viscosity
The unit of viscosity can be expressed as Pa s (Pascal second)
The experiment you have done equates this viscous drag force with the weight of the falling object. The two forces balance at the terminal velocity.
You may also need to take into account the buoyancy upthrust.
[ignoring the buoyancy term]
[including the upthrust. V is volume of marble, is density of water]

From which you find the viscosity by measuring the other quantities.
v is the terminal velocity.
3. thank you! that makes things seem a lot clearer!!
which type of viscosity is this ... kinematic, dynamic or a general viscosity coefficient?
thanks
4. also ... I have seen a few other formulas.
can you explain the difference with the one you gave and a few others ...

http://i17.photobucket.com/albums/b6.../viscosity.jpg,
http://www.engr.iupui.edu/me/courses...xperiment8.pdf (page 3)

are they used to determine different sorts of viscosities?
many thanks
5. (Original post by sc_spurs)
also ... I have seen a few other formulas.
can you explain the difference with the one you gave and a few others ...

http://i17.photobucket.com/albums/b6.../viscosity.jpg,
http://www.engr.iupui.edu/me/courses...xperiment8.pdf (page 3)

are they used to determine different sorts of viscosities?
many thanks
Kinematic viscosity is just what you get when you divide absolute or dynamic viscosity by the density of the fluid.
You are dealing with absolute (or dynamic) viscosity. They are the same thing. That's what the term in the equation is.
The formula in your 1st link is what you get if you rearrange the (2nd) one I gave you to give viscosity as the subject, and use the volume (4/3 pi r³ ) and density of the marble to find its mass. You also use the volume of the marble to find the upthrust due to the weight of water displaced - Archimedes Principle.
This formula only works for what is called "steady" flow. That is to say, the marble shouldn't fall too quickly such that the flow around it is turbulent. (It's what is called "lamina" flow.)
Most A Level books should give a description of this experiment, including the correct formula.
The other link you gave looks like a far more advanced method (it's an engineering text) which takes into account some of the inaccuracies in the school lab method. I wouldn't worry about it. Ignore it.
6. ok ... thanks for the help!
just to confirm, with your equation ..
viscosity: Pascal second
density: kg/m^3 (i have seen some with g/m^3)
mass: kg (again, seen some in g)
volume of marble: m^3 (again ... some american versions use all sorts)
g: 9.8 (i have seen some where they use 9800 and cm not m)
thanks
7. (Original post by sc_spurs)
ok ... thanks for the help!
just to confirm, with your equation ..
viscosity: Pascal second
density: kg/m^3 (i have seen some with g/m^3)
mass: kg (again, seen some in g)
volume of marble: m^3 (again ... some american versions use all sorts)
g: 9.8 (i have seen some where they use 9800 and cm not m)
thanks
Keep everything in SI units. kg, m, s, N, Pa
No cm or g.
Avoid American sites if you can.
The physics is excellent, but if you do go there - watch the units. You will find all sorts, including feet and pounds. [Some British sites, particularly engineering related, use those too, by the way!]
The point is, the system used by the exam boards in the UK is always SI.
8. that disussion was kinda cool.I thnk I already like this site.can use such help tips in my demanding course work of... Bsc in meteorology
9. wicked, thanks Stonebridge, seriously you have helped me no end!

hopefully I am all set, I will let you know later on today how it goes when I go about working out all my viscosities!
10. (Original post by sc_spurs)
wicked, thanks Stonebridge, seriously you have helped me no end!

hopefully I am all set, I will let you know later on today how it goes when I go about working out all my viscosities!
Good luck.
11. Hi, I have completed the experiment, yet now trying to work out a decent viscosity is proving a nightmare, and using my calculator and also http://goo.gl/dt71T, my results for viscosity are always way off. Is there any chance anyone could look over these figures and help me work out which part of my experiment has gone so horribly wrong?!

Ball, diameter 2cm, mass 30g, density roughly 7000kg/m^3
Time to fall (in seconds) 0.8m (80cm) in a tube of the following liquids (time is an average of 10)

Water 1.293
Veg Oil 2.617
Olive Oil 2.792
Motor Oil 4.99
Glycerol 20.77
Syrup 794.85

Corresponding Fluid density
998
915
921
1250
888
1495

The tube diameter is 2.5cm, so is this just too thin compared to the ball for a reliable measure to be calculated?
Thanks
12. (Original post by sc_spurs)
The tube diameter is 2.5cm, so is this just too thin compared to the ball for a reliable measure to be calculated?
Thanks
Unfortunately, it seems so. Take a look at this brief text book description of the experiment to see what they suggest.

13. damn, thanks for the extract, quite informative, everything else in the experiment was perfect, apart from the width of the tube
do you reckon there is some sort of constant that I can use to allow my results to somehow be relevant, or perhaps a correction formula? Or shall I go about inventing/discovering one? lol
14. btw, i don't suppose anyone has ever done a similar experiment, or knows where I can find the results of one online (i've done a bit of searching), just something to compare final results with
thanks
15. (Original post by sc_spurs)
damn, thanks for the extract, quite informative, everything else in the experiment was perfect, apart from the width of the tube
do you reckon there is some sort of constant that I can use to allow my results to somehow be relevant, or perhaps a correction formula? Or shall I go about inventing/discovering one? lol
Yes there is a correction formula. I managed to find it too.
However, I don't think it's a good idea to go this route in an A Level experiment.
If you are curious the formula is

where is the corrected value for viscosity for a sphere diameter d in a tube diameter D
I managed to find a reference here [pdf file]
http://www.engr.uky.edu/~egr101/ml/ML3.pdf
but it is a more advanced treatment aimed at engineering undergrad students.

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