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Shuttle/Mir Banner
microgravity logo Activity 6

Inertial Balance

Part 1

OBJECTIVE:

To demonstrate how mass can be measured in microgravity.

BACKGROUND:

illustration of setup
The microgravity environment of an orbiting Space Shuttle or space station presents many research challenges for scientists. One of these challenges is the measurement of the mass of experiment samples and subjects. In life sciences research, for example, nutrition studies of astronauts in orbit may require daily monitoring of an astronaut's mass. In materials science research, it may be desirable to determine how the mass of a growing crystal changes daily. To meet these needs, an accurate measurement of mass is vital.

On Earth, mass measurement is simple. The samples and subjects are measured on a scale or beam balance. Calibrated springs in scales are compressed to derive the needed measurement. Beam balances measure an unknown mass by comparison to a known mass (kilogram weights). In both of these methods, the measurement is dependent upon the force produced by Earth's gravitational pull.

In space, neither method works because of the free fall condition of orbit. However, a third method for mass measurement is possible using the principle of inertia. Inertia is the property of matter that causes it to resist acceleration. The amount of resistance to acceleration is directly proportional to the object's mass.

To measure mass in space, scientists use an inertial balance. An inertial balance is a spring device that vibrates the subject or sample being measured. The frequency of the vibration will vary with the mass of the object and the stiffness of the spring (in this diagram, the yardstick). For a given spring, an object with greater mass will vibrate more slowly than an object with less mass. The object to be measured is placed in the inertial balance, and a spring mechanism starts the vibration. The time needed to complete a given number of cycles is measured, and the mass of the object is calculated.

 

MATERIALS NEEDED:
Metal yardstick*
2 C-clamps*
Plastic 35mm film canister
Pillow foam (cut in plug shape to fit
   canister)
Masking tpae
Wood blocks
2 bots and nuts
Drill and bit
Coins or other objects to be measured
Graph paper, ruler and pencil
Pennies and nickels
Stopwatch
   *Available from hardware store

PROCEDURE:

Step 1. Using the drill and bit to make the necessary holes, bolt two blocks of wood to the opposite sides of one end of the steel yard stick.
Step 2. Tape an empty plastic film canister to the opposite end of the yardstick. Insert the foam plug.
Step 3. Anchor the wood block end of the inertial balance to a table top with C-clamps. The other end of the yard stick should be free to swing from side to side.
Step 4. Calibrate the inertial balance by placing objects of known mass (pennies) in the sample bucket (canister with foam plug). Begin with just the bucket. Push the end of the yard stick to one side and release it. Using a stopwatch or clock with a second hand, time how long it takes for the stick to complete 25 cycles.
Step 5. Plot the time on a graph above the value of 0. (See sample graph.)

Step 6. Place a single penny in the bucket. Use the foam to anchor the penny so that it does not move inside the bucket. Any movement of the sample mass will result in an error (oscillations of the mass can cause a damping effect). Measure the time needed to complete 25 cycles. Plot the number over the value of 1 on the graph.
Step 7. Repeat the procedure for different numbers of pennies up to 10.
Step 8. Draw a curve on the graph through the plotted points.
Step 9. Place a nickel (object of unknown mass) in the bucket and measure the time required for 25 cycles. Find the horizontal line that represents the number of vibrations for the nickel. Follow the line until it intersects the graph plot. Follow a vertical line from that point on the plot to the penny scale at the bottom of the graph. This will give the mass of the nickel in "penny" units.
sample graph
QUESTIONS:

  1. Does the length of the ruler make a difference in the results?
  2. What are some of the possible sources of error in measuring the cycles?
  3. Why is it important to use foam to anchor the pennies in the bucket?


 
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