Ask A Scientist© General Science Archive

name         Kristi
status       student
age          12

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)
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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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Kristi,

I'll try to simplify the definitions of these terms for you.  If what I say
doesn't make sense, email me back, and I'll try again.

When you looked up viscosity, you may have found a definition something like
this:  When a fluid is subjected to external forces, it resists flow due to
internal friction. Viscosity is a measure of this internal friction.

What this is saying, basically, is that viscosity measures how thick a
liquid is.  For example, if you pour water from one glass to another, it
does so very quickly and easily, but something like syrup, doesn't pour
well.  So, syrup is more viscous than water.  Viscosity is the term we use
when scientists measure this thickness, just like we use meters or inches to
measure length, or seconds to measure time.

You can try pouring different things to see how "thick" or "viscous" they
are.  Water, rubbing alcohol, bleach, are all things that pour very easily
and have low viscosity.  Syrup, ketchup, ranch dressing, pudding, and
mayonaise are all things that don't pour as easily, and have a higher
viscosity.


For density you might have found a definition like this: Density is the mass
of a substance per unit volume.

What this is saying is that, if you take the same amount of two different
things, things that are denser will weigh more.  For instance, if you take a
cup of bbs (for a bb gun) and a cup of water or a cup of olives, the bbs
will weigh more than the other things.  So, we would say, the bbs have the
highest density.  You could try this at home with a very good scale and
measure cups of different things.  Things that weigh more (if you have
exactly 1 cup of each) have a higher density than things that weigh less.
Or you could take different types of blocks, all cut to be exactly the same
size.  The different blocks would weigh different amounts.  Balsa wood
(which is found in most hobby stores), would be extremely light, and
something like Oak, would weigh much more.  If you had a block of lead that
was the same size, it would probably weigh the most, because it is very
dense.

So density, is closely related to weight.  We measure density by giving a
specific size of each object (called volume), and measuring how much of that
thing there (weight).


Relative density is simply how the densities of two different things relate
to each other.  Once you have measured the density of two different things,
relative density is a measure of how different the densities are.

An easy way to think of this is in terms of things that float or do not
float in water.  If you have a solid object (not hollow at all), and were
put it in a glass of water and hold it under the surface and then let go,
some of the objects would float, and some of the objects would sink.

Those objects that sink, have a higher density relative to water, and those
that float have a lower density relative to water.  This can be done with
many different types of things, such as wood, oil, metal, and plastic, and
for each thing you can see the relative density of that object and water.

Relative density is a little different from the other two types of
measurement--viscosity and density.  Both viscosity and density can be
measured using just one substance.  But relative density requires you to use
two different things, find the density of each, and figure out which has the
higher density.

I hope this simplifies things for you.

Thanks,
Eric Tolman
Computer Scientist
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Density is a measure of how much mass or "stuff" something has per unit
volume of "stuff".  Volume is a measure of how high, deep, and wide
something is.  Think of water that is in a gallon container.  Water is the
stuff and it has a mass, that is, it is a liquid contained in a container.
I have the volume of 1 gallon (because I said it was a gallon container).
So, the density of my water would then be mass of water/1 gallon to give me
the units of mass as mass per gallon.  Mass is usually measured in pound
mass (lbm) or kilograms (kg), so what you see in most books as units for
mass are lbm/ft^3 or lbm/in^3 or kg/cm^3.
Relative density is the measure of the density of something compared to the
density of something else.  Most densities of liquids are compare to the
density of water, and this is called specific gravity.  So, if you know the
specific gravity of a fluid, and you already know the density of water, you
can find the density of that fluid by multiplying the density of water times
the specific gravity.  That is just one example of relative density.
Viscosity is the measure of a fluids resistance to flow. Imagine I have a
wood block floating in a very still bath tub filled with water.  If I give
the piece of wood a slight push, it will move to one end of the tub to the
other.  Now, if I drain the tub and put in a higher viscosity fluid (say,
honey) and let the wood float in it again.  If I push the wood with the same
force as I did when it was in water, the piece of wood will only go to the
middle of the tub.  Because the honey has a higher viscosity, it has a
higher resistance to letting something move through it.  Air is not very
viscous, so you can walk through it pretty easily.  When you are in the
swimming pool up to you chest, it is very hard to walk through because water
is more viscous that air.
Hope these explanations help.  Good luck in the science fair.

C. Murphy
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Viscosity is how gooey a liquid is.  so pancake syrup is very "viscous", it
doesn't flow very well.   same with a thick milk shake.  but water flows
pretty well if you pour it.

density is sort of like what floats and what doesn't.  something that is
big, but not all that heavy is NOT dense.  something that is small, and
really weighs alot is dense.
So the toys you used to play with with the swimming pool, or bath tub are
not very dense.  But a brick is -- you throw it in water, it sinks.  It is
more dense than water.

Relative density just means how dense is one thing compared to another.  The
brink sinks in water, so it is relatively more dense.

Ross
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