EFFECTS OF WEIGHTLESSNESS
The Space Shuttle continues in a state
of constant free fall until it maneuvers back to Earth. This free
fall is the source of the "apparent weightlessness"
which astronauts experience in orbit.
Some of the very real effects of this "apparent
1. Loss of bone mass (similar to osteoporosis)
2. Reduced total blood volume, particularly loss of red blood
3. Giddy, light-headed feeling
4. Space sickness with nausea and vomiting
5. Decrease of heart size
6. Nasal congestion
7. Muscle weakness
Note that the heart does not have to work
as hard in space to pump blood. On the other hand, the heart must
work hard on the ground because it must pump blood against the
force of gravity.
Similarly, muscles do not need to work
as hard in space due to the apparent lack of gravity.
Astronauts can maintain healthy muscles
in space by exercising. For example, astronauts aboard MIR exercise
using a treadmill and stationary bike.
Another effect that astronauts may experience
is possible tissue damage from radiation. There is no atmosphere
or ozone layer to protect the astronauts from this radiation.
There are two types of weight:
1. True weight
2. Apparent weight
"True weight" results from Newton's
law of gravitation.
The force F between any two particles having
masses m1 and m2 separated by a distance r is an attraction acting
along the line joining the particles. This force has the magnitude
F = G (m1)(m2)/ (r^2)
where G is a universal constant having
the same value for all pairs of particles.
G=6.6720 [10^(-11)] N m^2/kg^2
Reference: Halliday & Resnick, Physics
Parts 1 & 2, Wiley, New York, 1978.
A particle can be a planet, a star, a person
standing on a planet, or any physical object whatsoever.
The formula for the true weight W can be
derived from the Newton formula:
W = mg
where m is the object's mass and g is the
acceleration of gravity. The acceleration of gravity at the Earth's
surface is about 9.81 meters/sec^2. Again, the "true weight"
does not depend on an object's state of rest or motion.
"Apparent weight" is essentially
the weight measured by placing the object on a bathroom-type weight
scale. Imagine that you were standing on a weight scale inside
an elevator. The scale would show that your "apparent weight"
increases as the elevator accelerates upward. On the other hand,
your "apparent weight" would decrease as the elevator
An object in a free-fall has "zero
Space shuttle astronauts experience "apparent
weightlessness" as the shuttle orbits the Earth. Nevertheless,
this "apparent" condition produces very real physiological
effects, such as loss of bone mass.
Space shuttle astronauts cannot use a bathroom
scale to measure their weight. William Pogue was an astronaut
on the Skylab space station. He wrote that this space station
had a special chair that swung back and forth on springs. This
device was called the body mass measurement device (BMMD).
The mass of the person sitting in the chair
could be calculated from the period of oscillation. The period
is the time required for one complete cycle of back-and-forth
The person's "true weight" could
then be calculated by multiplying mass by the gravitational acceleration.
Please send comments and questions to Tom
Irvine at: firstname.lastname@example.org