Some
background info first. Mass and weight are two entirely different
things with very specific meanings in physics, (the branch of science
concerned with forces, waves and motion.) Both mass (as in "massive",
not the thing you go to on Sundays) is the amount of "stuff" in an
object whereas weight is a force. Mass is the same no matter where an
object is located in the Universe, however weight is a force due to
gravity. On planets where gravity is less, objects weigh less and vice
versa (This is why astronauts appeared to bounce around on the Moon.)
Mass and weight are both measured in kilos, and the kilo, along with the
metre, second and others is one of the seven base SI units. (SI stands
for the French Système International)
How is weight measured?
A
traditional weighing scales uses a spring in its mechanism for working
out weight. As the spring stretches or compresses due to weight being
placed on a connected pan or hook, a pointer attached to the spring
moves on a scale, indicating weight. Another technique for measuring
weight is to use a balance. These were regularly used in grocery and
hardware stores for measuring food items, fertiliser and ironmongery
such as nails. The idea is simple: A balance is like a seesaw. Place the
material to be measured on one side and stack up a series of assorted
known weights on the other side until the "seesaw" is balanced and a
pointer indicates zero. Then the weights on both sides are the same.
More modern weighing scales are electronic, using piezoelectric load
cells that produce a voltage proportional to weight. This voltage signal
can ultimately be used to drive a digital display.
Why can't astronauts use "normal" weighing scales?
In
space astronauts are weightless. Actually that's not strictly true,
they're just constantly falling which makes them weightless, rather than
truly weightless as they would be in deep space, far away from any
planets or stars. The fact that they're weightless means they can't
stand on a normal scales and press down on it. So a different technique
must be used as explained by Canadian Space Agency astronaut David
Saint-Jacques in the YouTube video below.
