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What is the unit for viscosity when using this equation?

Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.

Bess Ruff, MA

Environmental Scientist

Expert Answer

The unit of measurement used in this equation is a pascal second (Pa s).

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Question

What is the viscosity of water?

Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.

Bess Ruff, MA

Environmental Scientist

Expert Answer

At 25 degrees Celsius, the viscosity of water is 8.90×10−4 Pa s.

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Question

Is viscosity directly proportional to the density of the liquid? If so, is there any chart to calculate the viscosity?

Community Answer

Viscosity and density relate to two different properties of a liquid. The former has to do with shear effects between molecules, and the latter with molecular mass and how closely those molecules are packed.

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Question

How do I time the fall through an opaque liquid?

Community Answer

It would probably be difficult unless the sphere has a bright color and can be seen through the liquid. Since the result most likely won't be as exact, it would be a good idea to choose a different method to measure the viscosity.

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Question

If the formula presented for viscosity only holds if the sphere has reached its terminal velocity, how do I know how far from the surface this happens?

Community Answer

The reference is given in the step number 4, the explanation will be as follow, when you have friction or drag produced by the travel medium (liquid in this case), the object drag is produced in the area thad directly touches the medium while in travel, since this experiment is based on a sphere, and without considering rotation or wall distances, the theoretical friction is produced in the leading half on the direction of the motion, once you pass that point and the ball is covered with liquid, the friction becomes constant (since the acceleration is given by gravity), and at that point you have achieved max speed. That is the reasoning to suggest a 2.5 cm mark below the liquid level.

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Question

When a unit is measured in "cP," what does this mean?

Community Answer

It means the unit of measurement is centipose. 1 Pa s = 1000 cP roughly. Both units are correct. The formula you use will determine the units, but you can easily convert them either way.

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Question

What does the "A" mean in the viscosity equation?

Community Answer

That's the radius of the sphere. It tells you how to measure that in step 2 above.

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Question

Should all the units be the same when measuring viscosity?

Community Answer

They should be. Hence the above formula, which uses g/mL instead of kg/cubicmeter, is not intuitive. However, the final answer appears to be close to the expected value.

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