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8th Grade Physical
Science Curriculum
Unit 1: Density
and Buoyancy
Lesson 4. Fluffy vs.
Not Fluffy
Objectives:
Students will be able to
visualize the difference between a low-density and high-density
material. Students will be able to demonstrate, in some way,
how the mathematical expression for density describes the difference
between "fluffy" and "not-fluffy."
Key Questions:
What does a gram of cotton
look like? A gram of sand? How much does a cup of cotton weigh?
Is one gram of Cotton balls more than one gram of sand? What
does "more" really mean in this case?
Overview:
Students will have measured
the density of several liquids by now. However, they are likely
still to have difficulty visualizing density as the ratio
of the mass of a substance to its volume. This activity is designed
to help the students visualize the effect of density and to realize
that both mass ("weight") and volume must be considered
in concert with one another.
In the first part of this
activity the students will weigh one gram of several solids
that have very different densities ("fluffy" stuff
and "not-so fluffy" stuff.) They will then reflect
on what they observe, on what challenges they had in weighing
the materials and what they can say about the volume of
the materials they have weighed.
In the second part of the
activity, they will measure approximately equal volumes
of "fluffy" and "not-so fluffy" solids and
compare their masses ("weights.") Again, they will
reflect on their observations through class discussion, Venn
diagrams, etc.
Materials:
Handouts with tables and
instructions for students
For each group of 4 students:
Balance (triple beam)
3 or 4 Plastic "Solo" cups per group
3 or 4 Small paper cups (optional)
Materials to be weighed:
Bag of cotton balls
Bag of packing "peanuts"
Salt
Sand
Note: Have enough sand or salt to fill either a small
paper cup or the Solo cup for Part 2. Students will measure either
sand or salt.
Procedure: Have students record their data in a laboratory
notebook. You may wish to provide them with a table like the
one following the procedure -- or -- have them make a table in
their notebooks.
Part 1: One gram of "Fluffy
Stuff" and "Not-so Fluffy stuff
Have one set of the supplies and materials listed above available
for each group of four students. Instruct the students to weigh
one gram of one of the "fluffy" materials. (alternatively,
you may wish to assign specific materials to each group.) Ask
them to think about how they will hold their materials together
while they weigh them. Some may wish to use the Solo cups to
hold the material. Ask them questions such as: "How will
you know when you have one gram of the "fluffy stuff?"
Encourage them to write down everything
they try and observe in their science notebooks. Let them grapple
with how to take the mass of the cup into account (the "tare"
mass of the solo cup.) They may have trouble fitting one gram
of cotton balls into the cup. Again, encourage them to design
a solution to their problem.
Once they have weighed the "fluffy"
materials, instruct them to weigh one gram of the sand, one gram
of the salt and one gram of paper. Encourage them to write down
all of their observations and the solutions to any measurement
problems that they encounter. Ask them about what problems they
might encounter if they put the sand or salt directly on the
balance pan. Encourage them to think about weighing their sand
or salt onto a piece of paper or into a Solo cup. Again, they
will have to "tare" the paper or the cup.
Observations and Reflections:
After the students have
weighed one gram of each material, have them discuss in small
groups or with the whole class some of the following:
° What did they notice about
the amount of each material? How are they the same?
How are they different? Try showing the similarities and differences
on a VENN diagram.
° What challenges did they have
to solve in weighing each substance?
After some class reflection, continue
with Part 2, in which the students compare equal volumes
of different materials.
Part 2: Equal volumes of different
solids:
Ask the students to fill a small
paper or Solo cup with packing "peanuts." Or cotton
balls. Have them fill a second identical cup with sand (or salt.)
Instruct them to write their observation in their notebook (suggest
that they hold each cup, and compare them in any way they might
wish.) Then instruct them to weigh each cup and record the masses
in their notebooks. You may use the sample Table for Part 2 or
have the students create their own in their notebooks.
Observations and Reflections:
Ask the students questions
such as: "What can you say about the volume of each
substance? What can they say about the mass of each substance?"
Have them think about what is similar and what is different about
each material that they have measured. Have the students describe
what they mean by the "volume" of a material.
Ask them to discuss the word "amount" in terms of their
measurements and observations. Is "amount" an accurate
word for any of the measurements they have done? Why or why not?
Ask the students to compare the amounts of each material.
Ask them: "How are they the same? How are they different?"
Suggest that they show the similarities and differences on a
VENN diagram.
Ask what they can say about the volume of each substance.
What can they say about the mass of each substance for
this part of the activity? Again, have the students describe
what they mean by the volume of a material compared
to its mass.
Ask them how the density of each material is related to
what they have observed about its volume and mass. Encourage
them to describe the differences and to attempt to express
it mathematically
Have the students write their reflections about mass, volume
and density in their science notebooks.
Assessment Suggestions: At this point in the unit, you should look
for conceptual gaps and expect a beginning of understanding that
the mass, volume and density of materials are all related to
one another. Some suggestions:
1) The homework assignment can be one assessment.
2) Another (or alternative) assessment at this point could
be to ask student to write 2 - 3 questions about the mass, volume
and density of materials.
Homework:
1) Write a short paragraph
describing what they understand by the ideas of mass and volume.
Their paragraph should include:
a) Their idea of the meaning of mass and volume (NOT a
glossary or dictionary meaning),
b) Examples of materials with various combinations of
large and small masses and large and small volumes,
c) Their idea of what makes a solid "fluffy"
or "not-fluffy."
d) How they would measure masses and volumes.
AND /OR
2) Draw and label a picture or cartoon
that illustrates their of idea of mass and volume. Their
drawing should include:
a) Some way of showing the meaning of mass and volume,
b) Examples of materials with various combinations of
large and small masses and large and small volumes,
c) Their idea of what makes a solid "fluffy"
or "not-fluffy."
d) How they would measure masses and volumes.
Teaching Tip to help students grasp the density as mathematical
ratio of mass and volume. Encourage students to think of visual
ways to express the mathematical relationship. For example: One
might describe fluffy solids mathematically as:
| FLUFFY DENSITY= |
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| Not-so-fluffy
density (such as copper or lead) = |
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| "Medium"
density such as water |
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