Density

Density ...

Which is heavier?

Which is heavier .. A kilogram of lead, or a kilogram of feathers?

The answer is obviously .. "They both have the same weight." There is something different about them, but it is not their "heaviness".

How about this ... An astronaut weighs one-sixth their Earth-weight when they are on the moon. This is why they are able to jump so high when moving around. Their muscles are the same strength as on the Earth, but their weight is much less.

An astronaut is only one-sixth their Earth weight on the Moon.
Astronaut Eugene Cernan walks toward LRV during EVA -
Image: AS17-140-21388
Taken from: http://www.ksc.nasa.gov/mirrors/images/html/as17.htm

and this ... Have you ever tried to lift a friend when playing in water. They seem lighter don't they? Has their "weight" changed. Have they lost "weight" by being in water? If they have, why bother with diets?

Obviously they haven't 'lost' any of their body. Something about the word "weight" is causing a problem. This is why scientists use the term "Mass". The mass of an object doesn't change just because gravity changes. Mass doesn't change when an object is immersed in a liquid. Mass is a measure of the atoms that make up an object and that doesn't change.

The term that helps us understand the difference between the lead and feathers situation is Density. The density of an object is a measure of its Volume and Mass.

In the following 'experiments' one-centimeter cubed samples of four different metals are placed on a set of scales to find their masses.

copper.gif (2423 bytes)

From these results we can say that Copper
has a mass of 8.9 grams per cubic centimeter.

aluminium.gif (2376 bytes)

From these results we can say that Aluminium
has a mass of 2.6 grams per cubic centimeter.

lead.gif (2349 bytes)

From these results we can say that Lead
has a mass of 11.3 grams per cubic centimeter.

iron.gif (2352 bytes)

From these results we can say that Iron
has a mass of 7.2 grams per cubic centimeter.

The density of a substance is a statement about its mass per unit volume. The above results are actually the densities of the metals. Copper has a density of 8.9 grams per cubic centimeter, Aluminium has a density of 2.6 grams per cubic centimeter, Lead has a density of 11.3 grams per cubic centimeter and Iron has a density of 7.2 grams per cubic centimeter.

This technique is fine if you have a one-centimeter cubed sample to deal with. What happens if the samples are bigger, or irregular shapes? One approach is to use a measuring cylinder containing a liquid (usually water). The object is placed in the liquid and the change in water level is recorded. The change in level represents the volume of the object.

Finding the Density of an irregularly shaped object.

The object has a volume of 15 mls, or 15 cubic centimeters.

The object has a Mass of 98.5 grams.

Since

Density = Mass divided by Volume

the density of the object is 98.5 divided by 15 which equals 6.57 grams per cubic centimeter, or 6.57 g/cm3 which is slightly less than the density of iron.

Density - QUIZ

Your Name and School -

Do you Certify the answers below to be all your own work? -

1. Which is more dense - wood, or concrete? How do you know?

2. An astronaut weighs 120 kg on Earth. How much would they weigh on the Moon?

3. What would be the mass of ten cubic centimeters of Copper metal?

4. What is the volume of one tonne of Lead?

5. An object has a mass of 7 grams and a volume of 21 cubic centimeters. What is its density?

6. "Spent" Uranium metal was used in some bullets in the Gulf War. Why was Uranium used in this way?

7. Describe how you would work out your own density.

8. What is the URL of a Web page containing information about density?

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