Shape Change and Density
Date: Fall 2009
Why does changing the shape of an object have no effect on
the density of that object?
I have got a couple of answers that might help your thinking on
density. First of all, density is a characteristic physical
property of a substance. That means this characteristic physical
property can be used to identify one substance from another. For
example, boiling points and melting points of substances are usually
quite different from other substances. Heating a substance and
noting when it melted or boiled would give you a clue as to what the
substance is. Density is the same kind of characteristic physical
property. You could tell gold from brass if you could obtain a mass,
volume and do a little division. Density does not change for the
substance unless you somehow altar the substance on a molecular
level, or have a sample size near the molecular size.
Now, you could take a lump of lead and hit it with a hammer until it
became a thin sheet. The density of lead is still 11.3
g/cm3. Flattening the lead has not changed the mass or the volume
of the lead. Unless of course some of the lead broke off while you
were hitting it. However, the decrease in mass would coupled with a
decrease in volume. The ratio between these 2 factors remain the
same and the density remains unchanged. Think about a block of
aluminum and a piece of aluminum foil. The density of Al is still
2.7 g/cm3 whether its flat or a block. However you can do so much
more with that piece of foil. You could form a little boat out of
the foil and even get it to float on water. Hey that is crazy, the
density of Al is 2.7 g/cm3 and the density of water is 1.0 g/cm3,
this should sink! But you and I know it does not. Making a foil
boat has not changed the density of the boat; the aluminum still has
a density of 2.7 g/cm3 but now we're talking about buoyancy.
I am going to leave you with buoyancy to think about. I hope the
density part made sense. Write back if you need a jump start on any thing.
An item's density is its mass divided by its volume.
The units of density are: pounds per gallon (lbs/gal), grams / liter, and
whatever other units you want to measure density in.
So, since changing the shape of an object does not effect its mass or
volume, the object's density remains the same.
You might ask, what if I put one gallon of jet fuel in a two gallon tank.
The density remains unchanged because although the jet fuel is in a two
gallon tank, it still occupies one gallon of space and still weighs 6
pounds. If you added two gallons of jet fuel to the two gallon tank, you
will have 12 pounds of jet fuel in a 2 gallon tank which reduces to a
density of 6 pounds of jet fuel per gallon.
Another way to think of it is if you change the shape of a pillow by pushing
in on it, the pillow will grow a bulge in some other place, maintaining the
same weight and volume.
This URL gives a table of some common items along with a history of density:
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Update: June 2012