Like when you see the numbers bounce around through several graduations on the digital scale till it settles down.
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So on a digital scale, the circuit it uses is a pressure sensor. This is constructed in such a way that the current/voltage across a circuit changes based on how much pressure is being placed on part of the circuit.
That’s not really constant. Your body’s constantly making tiny adjustments to your posture to maintain balance. Our brain ignores these because they aren’t important to us. But the pressure sensor feels it.
Still, unless you’re doing something like lifting and lowering one foot the amount of pressure on the sensor stays within a relatively small range. So the scale is actually sampling lots of readings for a few seconds then displaying an average. When it takes longer, the fluctuations are bigger, so the scale is waiting until they seem to stay inside of a smaller range.
You can see it real-time on “analog” scales, the ones with a speedometer-like display. When you step on it the needle fluctuates a little before settling, and if you bend your knees then stand up you’ll see it move around a little. Once you sit still it can settle down but as long as you’re moving it’ll fluctuate. And if you could see real zoomed-in video if it, you’d note it never actually “sits still”, it just gets so stable we don’t notice it’s moving.
Essentially, because the scale isn’t measuring the mass of the thing on it, it’s measuring the force that’s pushing down and telling you the mass (weight) based on that.
You know that force is based on mass+acceleration, which is why a car going at 55 MPH into a wall makes a much bigger mess than the same car going at 1 MPH. Same difference between a fist hitting you fast vs being touched gently by a fist; or even a very light (in mass) bullet hitting something at supersonic speeds.
So when you step on the scale, what the scale is measuring is a combination of your mass AND the acceleration of your body in the downward direction. If you were completely immobile, the acceleration would only be from gravity and the force would be prety stable.
The shifting that you see comes from you not being completely immobile. So if you were to bend your legs for example, you are accelerating your legs up a little bit, working against your body’s momentum, and it takes a little bit of the force off the scale. Then as you stop bending your legs, the force goes up a little, and so on and so on… Every little bit of movement you do is changing the direction and amount of acceleration that is being sent directly down, which affects the force against the sensor, which it reports back to you.
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