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.
To demonstrate how mass can be measured in microgravity.
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
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.
Plastic 35mm film canister
Pillow foam (cut in plug shape to fit
2 bots and nuts
Drill and bit
Coins or other objects to be measured
Graph paper, ruler and pencil
Pennies and nickels
*Available from hardware store
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
| 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.
- Does the length of the ruler make a difference in the results?
- What are some of the possible sources of error in measuring the cycles?
- Why is it important to use foam to anchor the pennies in the bucket?