Final answer:
If you twist a thread with a weight attached and lift it off the ground while standing on a scale, the scale will show the total weight of you and the ball. Bathroom scales measure weight, but are calibrated to show mass based on Earth's gravity. In free fall, the scale would show an apparent weight of zero since no force is exerted on the scale's springs.
Step-by-step explanation:
When you stand on a scale with a weight, such as an iron ball, and you twist the thread so the weight lifts off the ground, you create tension in the thread which essentially makes the ball part of your system. The scale will measure the total force exerted on it by both you and the ball. However, if the cable of an elevator snaps and you are in free fall, the scale will not show your real weight, but apparent weight, which would be zero since both you and the scale are accelerating downwards at the same rate due to gravity.
Bathroom scales measure the force exerted on their springs by an object, which we perceive as weight. These scales are calibrated to display mass based on the assumption that you are on Earth and not accelerating. If you were to push down on a table while standing on the scale, the reading would increase because you are adding more force onto the scale.
Ultimately, scales measure weight but are calibrated in mass units, such as kilograms, by dividing the weight by the acceleration due to gravity, which is approximately 9.8 m/s2. The scale will not read the same mass on the Moon as on Earth because the acceleration due to gravity is different.