Measuring Mass: How can a balance be made?

(From Experiment Central, published by U·X·L and available in Science Resource Center, a Gale online database.)

Purpose/Hypothesis
A useful measurement for science is not the weight but the mass of an object. The mass is the amount of atoms that make up an object. Here is your hypothesis: "By creating a balance with counterweights, you will cancel out the effects of gravity and calculate the mass of an object."

The materials used as a counterweight can be varied if the mass is known. The balance you will create is accurate only for low–mass objects. Do not exceed .9 of an ounce (25 grams) or accuracy will diminish.

Level of Difficulty

Moderate.

Materials Needed

• two 5–ounce (148–ml) cups
• plastic ruler, 1 foot (30 cm) long
• dried beans
• quarter, penny, nickel
• 30 small metal paper clips
• pencil
• Optional: dried split peas, Popsicle sticks

Approximate Budget

• $4 for the beans and wood.

Timetable

Approximately 30 minutes.

Step–by–Step Instructions

1. Place the pencil on a level desk. If the pencil rolls, the desk is not level.
2. Mark the ruler in the middle.
3. Place the ruler over the pencil at right angles, as illustrated.
4. At each end of the balance place the 5-ounce (148 milliliter) paper cups. Draw rings to mark their positions.
5. Make sure the ruler is level, and neither side is touching the tabletop.
6. If a side is touching, very slightly move the ruler as it rests on the pencil. Try to balance it perfectly.
7. As a test material, place a quarter (.19 oz or 5.5 grams) in one cup.
8. Place 1 nickel (.175 oz. or 5 grams) and one paper clip (.018 oz. or 0.5 grams) in the other cup. The balance should be level.
9. Continue to test other combinations of materials to determine which have equal mass.

Below is a list of common materials and their mass:

• nickel: 175 oz. (5 grams)
• dime: .08 oz. (2.3 grams)
• penny: .087 oz. (2.5 grams)
• quarter: .19 oz. (5.5 grams)
• wooden Popsicle stick: .05 oz. (1.5 grams)
• one paper clip: .018 oz. (0.5 grams)
• dried split pea: .003 oz. (0.1 gram)

Troubleshooter's Guide

Problem: When you tested the quarter, it did not balance.
Possible cause: The balance is not accurate to the .5 gram point. The actual mass of the quarter is 5.6 grams. Try adding one or two split peas to counter the weight.

Problem: The balance keeps tipping and it does not seem to level out.
Possible cause: Try using a pencil that has flattened sides to decrease sensitivity.