From supermarket to super highways, scales often lie in wait, unseen until they are needed. As different in scale as they might be, the principles behind the device that weighs produce and the meter that weighs trucks are the same.
Many scales operate on the lever principle: If you apply loads across a fulcrum (or pivot point), they will balance out when the loads are equal. With weight scales (also called balances), as your target weights on one side approach the weight of the item you are weighing, the scale returns to balance.
Newton's Third Law of Motion states for every action, there is an opposite reaction. The spring principle is an extension of this law. The force of an item being weighed by a spring will cause the spring to stretch or compress (depending on the scale type) in a set ratio. By measuring the amount the spring moves, the weight can be calculated.
Keep in mind all the principles that make these scales work are functions that measure weight, not mass. Mass is a measure of resistance to inertia, not a measure of displacement due to gravity. The rule of thumb to separate the concepts of mass and weight is that if you need gravity to measure it, it's weight.
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