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Weightless in space, Thumping to talk

Q: Today on CNN, they reported that the astronaut and cosmonaut at the space station "lost weight" because of a restricted diet. Without gravity effects acting on a space station ("weightless"), how do they determine that the men had "lost weight"? (Gail, Charlottesville, Virginia)

Astronaut Loren Shriver keeps up his strength (with M&Ms) on the space shuttle [NASA]

A: Indeed. If an astronaut steps on the scales in space, he reads zero because he’s orbiting and, therefore, falling around Earth. But he knows he hasn’t lost that much "weight." His basic mass is still with him so that’s what he must measure. He does so with a Body Mass Measurement Device. More about this in a moment.

The weighing problem in space is the same I would face weighing myself while falling. Suppose I place a scale on a trap door, step on it, strap my feet to the scale, read the dial, and motion a pal to release the trap door. Whoosh! The scale and I drop into a deep well. Falling, I glance down at the scale. Zero reading.

Gravity is tugging me, though. Nothing, however, supports my falling scale or me. Normally, a floor pushes back on the scale with a force exactly equal to the force of gravity acting on the combined mass of my body and the scale. Now, the scale and I fall together. Likewise, the scale and the astronaut fall together. Both scales indicate zero.

So, the falling astronaut is stuck with measuring his mass, not his weight.

An astronaut onboard the space shuttle measures his “weight” — actually his mass — with a Body Mass Measurement Device. [National Space Biomedical Research Institute]

Each morning before breakfast aboard the Space Station, the astronaut clambers into the Body Mass Measuring Device (BMMD) chair. See figure. A fellow astronaut straps him in, cocks the deployment-and-release-device (which allows the seat to oscillate back and forth), and sets the timer. The astronaut holds a deep breath (to reduce reading jitter), his associate releases the seat, and the seat moves back and forth about three times as the timer records the oscillation period.

That’s enough to measure the period and, therefore, to compute his mass. Bingo! It’s done. He knows his mass this morning and thus can tell if he’s lost any mass, er, "weight."

If you’ve ever played with a Jack in the Box, you know how the BMMD works. You stuff Jack in the box, pushing against his spring to close the lid. When you trigger the lid, the compressed spring releases, Jack pops out and bobs up and down again (oscillates) a few times.

An idealized astronaut (mass, M), sitting between two springs (stiffness, K) — one compressed and the other stretched. The timer times his “chair” as it slides back and forth. [National Space Biomedical Research Institute]

The astronaut sits on a chair between two springs — one compressed and the other stretched. See figure. When he releases the chair, he goes back and forth a few times. The timer records the time it takes for one complete cycle of back and forth (the period, T). From that, and knowing the stiffness of the springs (K), he can compute his mass (M) in kilograms as follows:

M = KT/(2π), where π = 3.14159.

"This is an important device," says Kenneth M. Baldwin, professor of physiology and biophysics at the University of California at Irvine, "because it allows the astronauts to track their body mass, which is essential to normal performance."

Further Reading:

National Space Biomedical Research Institute: Measurement of muscle mass in space

Q: Do rabbits make sounds to communicate with other rabbits? (Amy, Saltburn, England)

A: A rabbit, normally silent amidst a world of hunters, makes many sounds to communicate.

Eastern cottontail rabbit quietly keeps an eye out for predators [William R. James, US Fish & Wildlife Service]

He thumps his hind leg — sometimes both together — to tell others of danger (remember Thumper in Bambi?). A terror stricken rabbit screams like a human baby as he flees a pouncing predator, is hurt, or dying. A doe coos softly somewhat like a dove but a deeper pitch, as she nurses her babies. A rabbit grunts to indicate anger.

The male honks — a low rhythmic sound — to court a female. Some females whimper loudly when courted. A male may growl briefly after mating.

A rabbit indicates contentment by vibrating his cheeks rhythmically to make a low chattering sound.

Further Reading:

Connecticut Department of Environmental Protection: Cottontail rabbits

The language of lagomorphs by Chandra Moira Beal

Stories rabbits tell by Susan E. Davis and Margo DeMello

(Answered Feb. 4, 2005)