This is regarding a digital bathroom scale; idk if it happens on analog ones, too. I’ve noticed that shifting my weight to one foot, to the balls of my feet vs my heels, etc. changes the weight readout–sometimes up to two pounds’ difference! So I’m curious to know why changing the weight distribution even a tiny bit affects the readout so drastically, when the same amount of weight is still on the scale.
In: 25
The weight isn’t evenly distributed along the scale. The plate you’re standing on isn’t directly tied to the “weight-o-meter” and it has some play. Because of that, if the center of mass you’re putting onto the plate isn’t directly above it’s center, the plate isn’t putting all of the weight of you onto the “weight-o-meter”
The scale isn’t usually measuring how much stuff is in you. It is measuring how much you’re pushing down.
Upward momentum. You can’t really “shift” your weight without using your muscles to lift part of yourself away from the scale. Since you’re not made of cinderblock, the stuff that makes you you continues moving after you’ve finished shifting, and it needs to start pressing down again for its weight to register.
Just imagine a very large sumo wrestler doing a side to side leg-thumping motion over a very large scale. His weight is on the scale the entire time, yet his shifting is causing the reading to change, similar as if you were to jump.
That and bathroom scales not being the most accurate devices in the world either.
The scale is measuring how much force you are putting in it. If you don’t stand in the middle of it (or rather with your weight pushing straight down) it will misread the force, because it’ll only register the straight downward component.
Depending on how exactly the scale is set up, this will lead to different errors.
Take for example a scale with 4 force sensors in the corners, if you just stand on one side of it, even worse on the outside of those sensors, you will twist the other side of the scale up, reducing the force in those sensors, while also not pushing straight down on the sensors if your side.
This is why highly accurate scales have an integrated water level: you need to adjust the feet of the scale to be perfectly level, so the force whatever you are weighing is putting down towards the center of earth is in exactly the same direction.
For human scales it really doesn’t matter, as a pound or two is well within daily fluctuations of body weight from food/drink/poop/urine.
You’d only need to make it perfect if you were doing some research where grams matter.
Which you could solve, if you were using a platform suspended on a string to a single force sensor it wouldn’t matter where in the platform you stand though, unless currently swinging around with the whole platform.
The scale is measuring the force you are exerting downward. If you shift your weight to one side or the other, it causes a moment on the plate in the scale and alters the force applied to the scale.
Probably due to where the sensors are placed.
Also consumer-grade scales are definitely a “get what you pay for” deal.
Torque.
It’s possible to temporarily convert some of your downward force (weight) into lateral force on the pressure sensors. These sensors are designed to measure force along one axis only, and so slightly off axis pressure (tilting the scale) will cause your weight to read as lower than it is.
Changes the centre of mass and causes a torque force that robs some of the downward force on the scale.
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