PPT
Density
Developer Notes
 Need exercises
 Objects in water - rock, cork, oil, Cartesian diver
 Objects in air - helium balloon, rock, regular air balloon
 People floating/ sinking in the ocean vs. fresh water, breathing in vs. out,
 Warm-up - How much does a helium balloon weigh? But this gets into buoyancy, which we
don't want to deal with here.
Version
02
Date
2004/04/07
Who
dk
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Revision
Updated to new format
Re-arranged some and added more material
Goals
 Students will know that density = mass/volume, D = m/V.
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Concepts & Skills Introduced
Area
Physics
Physiology
Concepts
Density
Time Required
Warm-up Questions
 Which weighs more, a ton of feathers or a ton of steel?
 Which is heavier steel or bone? (Indeterminable answer!)
 Would it be better to have steel bones? Why or why not?
Presentation
Although it may seem advantageous to have bones made out of steel, bones made of steel, but
the same size as regular bones, would be considerably heavier! Why? Bone and steel have
different densities. (The steel would also be stronger.)
The density of adult bone is about 1.7-2.0 g/cm3. The density of young bone is about .955-1.055
g/cm3. The density of steel is 7.8 g/cm3
Demo - Pass around objects that have the same volume but different masses and objects that
have different volumes but the same mass.
Demo - If you can get a mass set where the masses are the same diameter but varying length, you
can easily show that for the same material, as mass doubles, volume doubles, etc. It is a linear
relationship, and mass and volume are directly related.
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Assessment
Writing Prompts
Relevance
Answers to Exercises
Answers to Challenge/ extension
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Background
Problem
Materials
Procedure
Summary
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Reading
Why do some people float while others sink? What happens to our bones as we grow and age?
Why can birds fly while people can't? Although there are many factors involved, density is a
key. Density is a property of materials. It is determined by the masses of the atoms/molecules in
a material and the distances between them. It’s a measure of how much “stuff” is packed into a
certain space. Density = mass/volume
D=m/V
There is no separate unit for density like there is for force or pressure. The units for density are
just mass (kg, g) over volume (m3, cm3, mL, L). Density is usually measured in g/cm3. To
convert to kg/m3, multiply by 1,000.
Note: 1 mL = 1 cm3, 1 L = 1,000 mL = 1,000 cm3,
Density is affected by temperature, pressure and phase of matter.
Some people are less dense than others, so they float. Salt water is more dense than fresh water,
so people float more easily in the ocean than in lakes.
Our bones are not as dense when we're children; they can actually bend rather than break. Our
bones get denser in adulthood, but tend to thin out when we get old - that's osteoporosis.
Osteoporosis is the loss of minerals from bones - it makes them less dense and weaker. Many old
people break bones easily because of osteoporosis.
Exercises
1. How do you think the density of bird bones compares to human bones? Why?
2. What is the density of water in g/cm3 if 1 L of water weighs 1 kg?
3. The density of mercury is 13.6 g/mL. What is the mass of 5 mL of mercury?
4. Calculate the mass of a cube of copper with sides of 3 cm. Copper has a density of 64 g/cm3.
5. Calculate the density of a ball bearing that has a radius of 0.5 cm and a mass of 4 g. (Volume
of a sphere = 4/3 r3).
6. Ernest Rutherford was a great scientist who discovered that atoms have a dense center called
a nucleus. Rutherford used very thin sheets of gold foil in his experiments. Using the
following measurements, calculate the thickness of a piece of gold foil similar to that used by
Rutherford and his team.
a. Length: 17 cm
b. Width: 24 cm
c. Mass: 0.07 g
d. Density of gold = 19.3 g/cm3
Challenge/ extension
1. Why are birds smaller than land animals?
7. The mass of the sun is 1.99 x 1030 kg. The radius of the sun is 6.96 x 105 km. What is the
density of the sun in kg/m3?
2. In your organic chemistry laboratory you need to order more salt plates (small, solid disks of
NaCl) in order to finish the “IR specs” of an important unknown chemical. You forgot what
size disks you need to order (the catalogue lists them by mass). So, you decide to figure it
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Density
out. You know that the slot for the disk in your spectrometer is 9.0 mm thick and 22.5 mm
across. You also know that the density of solid NaCl is 2.165 g/cm3. Calculate the mass of
the proper size disk.
a. V=r2h (volume of a cylinder)
Glossary
Density - the mass per volume of a substance, D = m/V. (There is no unit for density.)
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