People ask, "How much do I weigh?" Then they run off looking for the scales. Our society is very concerned about weight.

Weight is measured using scales and weighing machines. It is measured in pounds and ounces or Kilograms and grams. What weight really measures is how strongly the gravity of your planet is pulling on you. For example, here is one of Dr Viau's students. She weighs 100 pounds on earth. However, when she goes to other planets, her weight changes because the planets have different gravities from the gravity on earth.

Mass is about how much material there actually is in an object. The mass of an object does not change so long as the object does not change. A bowling ball has the mass of a bowling ball no matter where it is in the universe.

Mass can be described in different ways. For instance, we can use the bowling ball as our standard.

Suppose that we decide to weigh a couple of cats.

Here, 2 cats = 1 bowling ball

Therefore, if the cats are the same size, as we see here,

1 cat has a mass of 0.5 bowling balls.

This is true anywhere in the universe if we use the same cats!.

Density compares the masses of different substances,

How do the masses compare with each other if you have the same sized sample of each?

Scientists take a cubic centimeter as their measure. This is roughly the size of a sugar cube.

Here you see some cubes with their densities.

You would need 3 cubic centimeters of water to get the same mass as 1 cubic centimeter of rock.

You would need 8 cubic centimeters of water to get the same mass as 1 cubic centimeter of iron.

Anything with a density of less than 1 (wood, styrofoam) will float in water.

So --

Melted rock would float on melted iron. This is why the core of the earth is made of iron, and why the continents, made mostly of rock, are on the crust.

The earth as a planet has a density of 5.52. This is because of the iron core. If the earth were just made of rock, its density would be close to 3.

Density is related to volume. Whatever fits into a cubic centimeter can be given a density. For example:

Egg whites in a bowl have a certain density.
If you beat the egg whites into fluffy peaks, the density of the beaten egg whites per cubic centimeter will be less than a cubic centimeter of the liquid egg whites in the bowl. The mass of the actual egg whites does not change, however.

Sometimes rock coming out of a volcano is full of gas bubbles. A volcanic rock called pumice has so many bubbles in it that it will actually float on water. The density of a cubic centimeter of pumice is less than the density of a cubic centimeter of bubble-free rock.

When you put air into your tires, you are increasing the density of the air in your tires to a density more than that of the air around us.

When you let hair-setting mousse out of a spray can, you can see the volume increasing. The density of the contents of the spray can becomes less when they are released, but the mass of the contents remains the same.

Out in space we see that stars can have the same masses but different densities.

At the ends of their lives, some stars shrink down into a sphere about the size of a basket ball! The matter in them is very, very dense.

Planets do not all have the same densities. They can be made of different materials: for example, the earth is made of rock and minerals, but Jupiter and Saturn are made of gases such as hydrogen and helium. Planets that are the same sizes may not have the same densities. If two planets are the same size, the denser planet will have a stronger gravitational pull. You would weigh more on the denser planet.