People rarely get to experience firsthand the phenomenon
of weightlessness, but that doesn’t keep SAT II Physics
from testing you on it. There is a popular misconception that astronauts
in satellites experience weightlessness because they are beyond
the reach of the Earth’s gravitational pull. If you already know
this isn’t the case, you’re in a good position to answer correctly
anything SAT II Physics may ask about weightlessness.
In order to understand how weightlessness works, let’s
look at the familiar experience of gaining and losing weight in
an elevator. Suppose you bring a bathroom scale into the elevator
with you to measure your weight.
When the elevator is at rest, the scale will read your
usual weight, W = mg,
where m is your mass. When the elevator
rises with an acceleration of g, you
will be distressed to read that your weight is now m(g + g)
= 2mg. If the elevator cable is cut
so that the elevator falls freely with an acceleration of –g,
then your weight will be m(g – g)
While in free fall in the elevator, if you were to take
a pen out of your pocket and “drop” it, it would just hover in the
air next to you. You, the pen, and the elevator are all falling
at the same rate, so you are all motionless relative to one another.
When objects are in free fall, we say that they experience weightlessness.
You’ve probably seen images of astronauts floating about in space
shuttles. This is not because they are free from the Earth’s gravitational
pull. Rather, their space shuttle is in orbit about the Earth, meaning
that it is in a perpetual free fall. Because they are in free fall,
the astronauts, like you in your falling elevator, experience weightlessness.
Weightless environments provide an interesting context
for testing Newton’s Laws. Newton’s First Law tells us that objects
maintain a constant velocity in the absence of a net force, but
we’re so used to being in an environment with gravity and friction
that we never really see this law working to its full effect. Astronauts,
on the other hand, have ample opportunity to play around with the
First Law. For example, say that a weightless astronaut is eating
lunch as he orbits the Earth in the space station. If the astronaut
releases his grasp on a tasty dehydrated strawberry, then the berry,
like your pen, floats in midair exactly where it was “dropped.”
The force of gravity exerted by the Earth on the strawberry causes
the strawberry to move in the same path as the spaceship. There
is no relative motion between the astronaut and the berry unless
the astronaut, or something else in the spaceship, exerts a net
force on the berry.