While weight changes with changes in gravitational pull, mass does not. If you were on the Moon, you could still accurately measure the mass of an object by using a balance scale instead of a spring scale. The different pull of gravity would still balance the mass of the object with the mass of the measuring weights on the other side. Could you accurately measure the mass of an object in free fall, where its weight did not register at all? If so, how?
Answer:
Yes, there are actually several ways, but the easiest is to use the same balance scale that you would use on Earth. The difference is that you would tie the scale to a rope, and swing it around in a circle, as shown in the photo below.
Inertia is proportional to mass, just as weight is. As you spin the scale in a circle, inertia makes it try to continue in a straight line, while the rope provides centripetal force to keep it from flying away. The inertia of the equal mass on each side will cause them to balance, even in freefall.
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