Measurement is an important part of any scientific experiment. In the past, scientists in different countries often used different units when measuring things. This made exchanging scientific knowledge more difficult. To solve this problem, a standard system of measurement was developed. This system is called the metric system.

In the metric system, the basic unit for measuring mass is the gram.

The Triple Beam Balance

A common instrument for measuring mass is called the triple beam balance. This device is found in classrooms, doctor's offices, and scientific laboratories around the world. The triple beam balance has the following key components:

Image showing the main components of a triple beam balance.

The balance has three beams called rider beams. Each rider beam has a different mass suspended from it. These masses, called riders, can be moved left and right along the rider beams. By moving the riders, you can determine the mass of an object placed on the measurement tray.

An adjustment knob is used to calibrate the balance. When no objects are sitting on the measurement tray, and all the riders are in their leftmost or 0 position, the pointer should be lined up with the zero mark. If it is not lined up the zero mark, you would need to turn the adjustment knob until it was. (Note that the triple beam balance in the Gizmo is always correctly calibrated, so you will not need to turn the adjustment knob. In the real world, however, balances should be periodically checked and recalibrated if necessary.)

  1. In the Gizmo, drag all three riders to their leftmost positions so that they are all set to 0 grams. If there are any objects on the measurement tray, return them to the Objects area.
    1. Drag the cube onto the measuring tray. What happens to the pointer? Why does this happen? (Hint: how is the triple beam balance like a seesaw?)
    2. Drag the cube off the measuring tray. What happens to the pointer now? Why does this happen?
    3. Drag the light bulb onto the tray. Does the pointer react in the same way as it did with the cube?
    4. Replace the light bulb with the paper clips. What is different about the way the pointer reacts? Why do you think this is?
    5. The cube has a known mass of 250 grams. Based on what you have observed so far, can you make any conclusions about the mass of the light bulb or the paper clips? Explain.
  2. Using a Triple Beam Balance

    To find the mass of an object using a triple beam balance, place the object on the measurement tray and adjust the positions of the three riders on the rider beams until the pointer lines up with the zero mark. The mass of the object can then be found by adding the values indicated by the three riders.

  3. Remove all objects from the measurement tray and set all riders to the 0 gram position. Next, place the cube on the measurement tray.
    1. Move the rider on the middle (500 gram) beam to the 100 gram position. Notice that the pointer remains above the zero mark. Move the same rider to the 200 gram position, and then check the pointer again. Since the pointer is still above the zero mark, continue moving the rider up 100 grams at a time until the pointer is no longer above the zero mark. What value is shown on the middle rider when this occurs?
    2. Move the rider on the middle beam back by 100 grams so that the pointer is above the zero mark again. Next, adjust the rider on the top (100 gram) beam until the pointer lines up with the zero mark. What value is shown on the top rider when this occurs? What value is shown on the middle rider? What is the sum of these two values? Is the mass written on the cube accurate?
    3. If the mass of the cube had been 254 grams instead of 250 grams, would you have also needed to adjust the rider on the bottom (10 gram) beam to find the cube's mass? Why or why not?
    4. Why do you think the riders have different sizes?
  4. Remove all objects from the Measurement Tray. Set all riders to the 0 gram setting. Try to determine the mass of the paper clips, light bulb, and cone by following the same procedure as in the previous step. Adjust the rider on the 500 gram beam, followed by the 100 gram beam, and then the 10 gram beam until the mass of each object has been determined.
    1. Use your data to calculate the combined mass of the cube and the light bulb. Check your answer by placing both of these objects on the measuring tray at the same time.
    2. Use you data to calculate the combined mass of the cone and the light bulb. Is it possible to check your answer using this triple beam balance? Explain.
  5. In each of the following situations, will the sum of the riders' values be greater than, less than, or equal to the mass on the measuring tray?
    1. When the pointer is above the zero mark.
    2. When the pointer is lined up with the zero mark.
    3. When the pointer is below the zero mark.
  6. Do you think you could determine the exact mass of a 220.47 g object using this triple beam balance? Explain what you might observe as you tried to determine the object's mass.