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)
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
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
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
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."
National Space Biomedical Research Institute: Measurement of muscle mass in
Q: Do rabbits make sounds to
communicate with other rabbits? (Amy, Saltburn, England)
A rabbit, normally silent amidst a world of hunters, makes many sounds to
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.
Connecticut Department of Environmental Protection: Cottontail rabbits
of lagomorphs by Chandra Moira Beal
Stories rabbits tell by Susan E. Davis and Margo DeMello
(Answered Feb. 4, 2005)