Contrary to popular belief, astronauts still have weight while
they are orbiting the earth. In fact, Shuttle astronauts weigh
almost as much in space as they do on the earth's surface. But
these astronauts are in free fall, together with their ship, and
their downward accelerations prevents them from measuring their
weights directly. Instead, astronauts make a different type of
measurement—one that accurately determines how much of them
there is: they measure their masses. Your weight is the force
that the earth's gravity exerts on you; your mass is the measure
of your inertia, how hard it is to make you accelerate. For deep
and interesting reasons, weight and mass are proportional to one
another at a given location, so measuring one quantity allows
you to determine the other. Instead of weighing themselves,
astronauts measure their masses. They make these mass
measurements with the help of a shaking device. They strap
themselves onto a machine that gently jiggles them back and
forth to see how much inertia t