Materials
1/2 meter stick *
six-sided pencil *
plastic cubes that have mass 1 gram *
lots of nickels (you provide)
other small objects that are easy to weigh: small rocks, a battery, ... (you provide)
plastic bag *
measuring cup *
water (you provide)

Mass is a property of an object, which doesn't depend on where it is. Weight is the gravitational force acting on an object. The actual size of the gravitational force varies by a few percent from place to place on the surface of the earth (and by more, elsewhere in the universe), but it is always proportional to the mass, so that two objects have the same mass if they have the same weight at the same place. Since mass and weight really are completely different things, it is appropriate to use completely different units to measure them. Force is measured in Newtons (as determined by the force scale), and mass is measured in grams or kilograms (which is 1000 grams).

One way to determine the mass of an object is to use a balance. The unknown object is put on one side and then objects of known masses are put on the other. The balance tilts towards the side with more mass on it. If you have a lot of objects of known mass, you can provide bounds on the mass of the unknown ("the mass of the unknown object is more that 17 paper clips and less than 18 paper clips"). If the two masses are very nearly the same, the balance might not tip at all (most of the time one side or the other wins)..

Please record this activity in your journal

1. Lay the 1/2 meter stick across an ordinary six-sided pencil, so that the 25 cm mark is right over the pencil. This makes a balancing stick. Use it to determine the mass of a nickel, by balancing one or two nickels (placed at the 1 cm mark) and a small pile of the plastic cubes.

2. Use the balancing stick, the cubes, and as many nickels as you have to determine the mass of some other small objects -- small rocks, for example -- until you have enough known masses to aggregate a mass of over 150 grams.

3. Put your collection of objects (that add up to more than 150 g) in a bag and use the force scale to determine how much your set of masses weigh (that is, the gravitational force measured in Newtons) at your location.

4. The gravitational force is proportional to the mass. Now that you know how large the force is for some mass, calculate the force on a 1 gram object and a 1 kilogram (=1000 grams) object. Note that the force scale is not sensitive enough to weigh 1 gram, and that it can't weigh objects as heavy as 1 kilogram.

Density

Some cooks measure out ingredients by weighing them, instead of using a volume measure (like a cup). This has some advantages: you can weigh out the flour before you sift it. It can be awkward translating a recipe written in one method into a recipe for the other, because each ingredient has a different equivalent: a cup of flour is 125 grams, while a cup of sugar is 225 grams. But we can use either method, because the mass is proportional to the volume for any particular material. This ratio is called the density:

density = mass/ volume

Density is a property of a material (rather than being a property of a particular lump of the material). We can ask, "Which has the greater density, flour or sugar?" because these are different materials and we would expect them to have different physical properties.

2. Determine the density of water and of the materials your nickels are made of. To do this, you will need to weigh a known volume of water (you can use the force scale, since you know how to convert the weight into a mass at your location), and measure or calculate the volume of a nickel. Here are some useful hints.

3. Nickels are 75% copper and 25% nickel. Down at the mint there are bars of this alloy. What is the mass and volume of a bar large enough to make a thousand dollars' worth of nickels?

Check the box when you are done:   Science Content: Force