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Name: Kristi
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 1999-2001


Question:
I would like to know what viscosity, density, and >> releative density is in words that I would be able to understand because the infoarmation that I found in the library is not clear enough for me to understand. (This information will be used fo my science fair project)


Replies:
Oh well, I am not sure if I can do better than the library but I try. Let's see. What is viscosity? Viscosity is a concept. It was created and used for telling how easy or difficult it is for a fluid to flow. As such, it is a measure of, or indicative of, a fluid's resistance to flow. Here is a simple experiment you can do. Dip a spoon in a glass of water, and another identical spoon in a jar of honey. Try to pull the spoons out AT THE SAME SPEED. Which one is harder to pull out? The honey, of course. So, we say that the viscosity of honey is higher than that of water, or that honey is more viscous than water.

Next, heat up the honey jar slightly and do the same experiment. You will see that upon heating, honey becomes more water-like, that is, less viscous. You need less force to pull out the spoon at the same speed. From this experiment we learn that viscosity of a fluid is affected by its temperature. Technically, viscosity is defined as the ratio of shear stress to shear rate. It unit is Poise. You'd say huh? To illustrate this complicated sentence, you can do this experiment. Take a pencil eraser (or a piece of rubber) and two pieces of wood, each twice the length of the eraser. Glue one piece of the wood to one side of the rubber and glue the other piece to the opposite face of the rubber. The pieces of wood must extend in opposite directions. Now, hold the ends of the wooden pieces and try the move them in the opposite directions but in line with the length of the rubber. The resistance of the rubber to be "sheared" (that is deformed in opposite directions) as you pull the wooden handles is a measure of its viscosity. How much force (shear) you need to apply to deform the rubber by a given amount in a given time (shear rate) tells you what the viscosity of rubber is. You also note that the concept of viscosity applies not only to fluids but also to plastics, rubber, and other things that can "flow" or deform.

Density is far simpler to describe. Suppose you have a carton of milk whose volume is one liter (1000 cc). If you fill it up with water and weigh it, it will be about 1 kg. So, the density of water is about 1 1kg/liter or 1000 kg/cubic meter. Now, if you empty the carton, and fill it up with honey and weigh it, it may be 1.5 kg (as an example). Thus, density, which is a measure of how "dens" something is, is 1.5 kg/liter or 1500 kilogram per cubic meter. Note that this has nothing to do with how viscous honey or milk is. You can fill the carton with sand and measure its density the same way.

Do viscosity and density ever mix up? No, never. They are unrelated to one another.

Why are people sometimes confused about these two? Well, there is a reason. In some engineering problems you have to consider both viscosity and density of a material. In those cases, a new term (concept) is coined called "kinematics viscosity." This is simply the viscosity of the substance (as discussed above) divided by its density. Why bother with this? Well, because you do not have to carry two numbers around in your calculations (for the same reason we say 2 instead of constantly saying 1 plus 1). Also, if you need to compare two fluids in your work, instead of comparing their viscosity and then their density, you can just use their kinematics viscosity and thus compare two instead of four numbers. Finally, relative density... that is real easy. It indicates how dense some material is with respect to another. Say, if you know that the density of honey is 1500 kg/cubic meter and the density of water is 1000 kg/cubic meter, you can say that honey is denser by a factor of 1.5. Or the relative density of honey is 1.5. Easy!

Good Luck

Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory



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