Samtheman1k said:
Erm, surely everything has an equal and opposite reaction? They should display the exact same weight.
Perhaps, IF the scales have a perfect mechanism. But they don't.
If you take a fairly high quality set of scales (say, the £1500 scales used at a supermarket checkout), and put a weight on them, then add a bit more, then add a bit more, up to the maximum the scale can read, then you progressively take the weights off again in the same amounts, you should get an identical set of readings, shouldn't you. But you won't. Or rather, the vast bulk of the time, you won't. It's called a linearity and hysteris test, and is part of the verification process that every commercial scale has to go thorugh before being certified as legal to trade.
If you put a calibrated weight, say
exactly 7.5Kg, onto the scale, then take it off, and repeat that three or four times, you should get a set of identical readings. But guess what? You wont, or nearly always won't. Again, it's part of the certification process. Oh, and the reading you get will also be likely to vary a bit depending on
where (front left, back centre, back right, dead centre, etc) you put that weight. That .... yup, you guessed, is another part of the certification process.
While doing that verification test, the verifier will also be looking for very small degrees of inaccuracy. If, for example, a store has an air conditioning vent blowing directly onto the scale, it's possible it'll be enough to prevent a valid reading. So can anyone leaning on the checkout, or walking by, or the store's door opening or closing, or ...... and so on.
Domestic scales are not made to anything remotely like the standard of these retail scales (let alone those used to measure drug quantities in a lab, for instance), and the mechanism is incredibly crude in comparison. For instance, you are extremely unlikely to be able to get a stable reading at all on one of these commercial scales, unless it is on a firm, stable (and usually level) base. Put it on a carpet and you quite possibly won't get a reading at all.
Put all that together, and you may get an idea of why a reading from a domestic scale is little more than a licked-finger in the wind estimate of weight. The mere fact that the person standing on the scale is incapable of standing perfectly still means that the scale mechanism MUST be resistant to variations caused by that movement, so when you step on it, it takes a spot reading at some point and, assuming it's digital, displays that. If it isn't digital, then in addition to the lack of refinement caused by the weighing mechanism (which will be electrical in those retail scales, taken by the change in resistance of the metal under stress in the load cell), you have gross inaccuracies caused by the needle, and the scale it's displayed against.
Basically, visage's hypothesis was correct, accompanied by the fact that the scale mechanism is, essentially, crap.
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