Eli5 how do scales work? How do we know the weight shown is accurate?
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Scales measure force. Gravity is an attractive force between two masses.
The mass of something you’d measure is much smaller than the mass of Earth, meaning the force imparted by Earth’s gravity is linearly related to mass for things smaller than the moon.
Gravity is variable across the Earth’s surface, because of differences in the globe’s elemental composition and elevation. Some highly accurate scales will let you program in the topological variation, from a map or from a calibration weight.
Which brings us to calibration. The kilogram is a standard unit of mass, a precise weight stored in a vault against which all others are measured. Copies of standards are then distributed to other countries. Recently, we have the ability to measure the masses of atoms themselves.
A scale can simply be a well constructed balance lever, up to piezoelectric force sensors, that electrically measure the compression of a crystal.
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