You know that Density is a measurement of Mass ÷ Volume.
Therefore Mass and Volume amounts affect the density.
If individual particles have more mass, density is affected: Styrofoam and gold.
If particles take up more space, density is affected: temperature and pressure.
You also know that pure substances have a specific density, no
matter how much of the substance you have.
But what happens when you have an object with a variety of
substances in it? For example, a boat.
This sailboat has rope, wood,
cloth, metal, plastic, etc.
Each of these individual parts
will have its own density.
Just as you have individual assignments in your courses
that have a total mark, objects in our daily life can
have individual parts that make up the total object.
When you talk about your “mark” in a course, you are
talking about the AVERAGE of all the parts calculated
together.
When we talk about the AVERAGE DENSITY of an
object, we are also talking about the average density of
all the parts calculated together.
The sailboat is designed to float, even when
it’s average density is changed………
What does this mean?
Well, when I add more rope, a few
lifejackets, and some people to the boat, I
change its’ mass. Now because I have
changed something in our Density formula, I
have changed the average density of the boat.
When I changed the average density, I also
changed how far down in the water my boat
will float or sink. There is a direct
relationship between density and floatation.
Can I “add” something to you to change your mass?
YES ! I can give you a chair to hold, I can give you a sack of
bricks to hold, etc. On a scale, your mass has changed.
Can I “add” something to you to change your volume?
YES AGAIN! When you put on clothing, especially a big winter
jacket, you are adding volume to yourself (check out how much
volume your winter jacket takes up in your locker).
So then, what does a lifejacket do? Talk about it with a
partner and explain it to me…….(fantastic test question by the
way!)*
Use some science vocabulary words: Average Density, Volume,
and Mass. Try this: Why a lifejacket and NOT a winter
coat?
A lifejacket adds very little mass to a person because of the
light-weight material it is made of and because it generally
does not absorb water.
However, it
having it on
your weight
volume that
adds a fair bit of volume to you compared to not
at all. The SIZE of a lifejacket is dependent upon
because the more weight you have, the more
will be necessary to change your density………….
Your AVERAGE DENSITY. (You are a combination of many pure
substances)
Most of the time when
you are wanting to work
with Average Density, it
is because you want
something to float or to
sink. You want to
affect the BUOYANCY
of an object.
So what types of “things”
control the rate of floating
and sinking?
Submarine
Scuba Diver
Jelly Fish
Fish
Shark (different!)
Next we’re going to get to the handouts to relate
Average Density to Water Displacement, and Buoyancy.