Lecture #24: Microgravity
What is weightlessness, really?
You’ve probably heard the terms
They all refer to the same sensation, which is what astronauts experience when they’re
either spacewalking or on board a spacecraft that’s in a natural orbit.
reasons, the preferred term among the NASA crowd for this type of environment is
Weightlessness is one of the most frequently misunderstood concepts in science, and so
we should spend some time getting a handle on it.
Most people think that astronauts are
weightless in space because there is no gravity there, or at least because Earth’s gravity is
very weak there.
this is not the case
In fact, the strength of Earth’s gravitational
pull on, say, the astronauts on the International Space Station in LEO, is only slightly less
than the pull we all feel here on Earth’s surface.
So right away, we need to recognize that
weightlessness is something that can be experienced
in the presence of gravity
gravity is a universal force, and there is no place in the universe that is entirely free of
way to experience weightlessness is in the presence of gravity.
So when do you feel weightless?
You feel weightless whenever gravity is the
force acting on you – i.e., when
you’re in a natural orbit.
Well, here goes.
First let’s make sure we understand what it means to experience
(as opposed to weightlessness).
Here on Earth, there is of course a force of gravity on
you which pulls you down toward the center of the planet.
But gravity alone is not
enough to generate the sensation of weight.
When you step on a bathroom scale, the
measure the strength of the Earth’s gravitational pull on you.
measures the compression of the springs inside the scale, which is due to the upward
between the bottoms of your feet and the scale.
The contact force is a
force you feel whenever some part of you is touching a surface.
While you’re standing
on the scale in Earth normal gravity (what we call 1-g), the upward contact force
precisely balances out the downward gravitational force, and so their magnitudes are
In this case, then, the scale
effectively reading your true weight.
(It is important
that the contact force be present – if it weren’t, the force of gravity would accelerate you
downward toward the center of the Earth.)
But we’ve all had the experience of feeling either heavier or lighter than usual.
happen on a high-speed elevator.
If you were to take a bathroom scale with you on the
elevator, it would measure the contact force on you – and that can change depending on
the acceleration of the elevator.
Suppose your true weight is 150 lbs.
When you’re at