PPT Density

Developer Notes

· 

Version / Date / Who / Revisions
01 / 2004/06/02 / dk / Initial version
02 / 2004/04/07 / dk / ·  Updated to new format
·  Re-arranged some and added more material
03 / 2004/06/09 / dk / ·  Added some more material.

Goals

1)  Students will know that density is a property of all matter.

2)  Students will know that density is force/area.

3)  Students will know and be able to use D = m/V.

4)  Students will know the difference between density and weight (heaviness).

Concepts & Skills Introduced

Area / Concept
physics / density, D = m/V

Standards Addressed

Time Required

Warm-up Question

·  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

Obtain three rods as follows:

·  A and B should be the same size (diameter and length), but different materials.

·  C should be the same material and diameter as B, but have the mass of A.

Aluminum and brass rod of the same diameter work well. The pieces don’t have to be rods, but if they have the same cross-section, then volume can be compared simply by measuring length. If not, you can always find volume by putting the pieces in water and measuring how much water overflows.

Start by handing each group an A and a C. Have them write the answer to the first question. They will generally say that the brass is heavier, when they mean it is denser. Let the answer wait until the end of the activity.

Now hand out B and have them fill in the table.

If you choose, you can have the students weigh (mass) the objects on a scale. It can be instructive to find the density of aluminum because it is an element.

Assessment

Writing Prompts

Relevance

Answers to Exercises

1) 

Answers to Challenge/ extension

1) 


Background

What makes something heavy?

Problem

Investigate some objects and find out more about their heaviness.

Materials

3 rods, labeled A, B, and C

1 scale

Procedure

Answer the following questions.

1)  Which is the heaviest, A, B, or C?

2)  Fill in the table. For example, if A has the most mass, write “A” in the first box. If A and B are the same (you can’t tell the difference), write “same”.

Compare / Mass (m) / Volume (V) / Ratio of m/V
A & B
A & C
B C

Summary

1)  Explain how the ratios of m/V make sense, since B and C are made of the same material.


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. All matter has mass. All matter takes up space. The amount of matter and space differs for each material, so we can compare their ratio, mass/volume. The ratio 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

When most people say one object is heavier than another, they mean it is denser.

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.

Density is affected by temperature, pressure and phase of matter. In general, materials get less dense (expand) as they are heated, and they get more dense (contract) as they cool.

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)

Here is a table of the density of some common materials:

Material / Density
kg/m3
Helium / 0.179
Air / 1.29
Water (4°C) / 1,000
Sea water / 1,025
Ice / 917
Wood (varies) / 600
Bone (varies) / 1,800
Aluminum / 2,700
Iron and steel / 7,800
Lead / 11,300
Gold / 19,300

Exercises

1)  How do you think the density of bird bones compares to human bones? Why?

1)  What is the density of water in g/cm3 if 1 L of water weighs 1 kg?

2)  The density of mercury is 13.6 g/mL. What is the mass of 5 mL of mercury?

3)  Calculate the mass of a cube of copper with sides of 3 cm. Copper has a density of 64 g/cm3.

4)  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 pr3).

5)  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

6)  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?

8)  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 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=pr2h (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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