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So, I wondered if my electrical weighing scale could detect the air pressure exerted by something flat like a plastic plate or even my hand. I noticed that if I move my hand with my arm vertical to the scale, 2-3 grams are detected at most, as expected since I move air toward the scale. If I try this with my arm parallel to the scale I can get it to detect -1 gram, if I do it quickly enough. I thought that maybe this happens because I "move away" some air from above it/decrease pressure or something?

Arm vertical:

enter image description here

Arm parallel: enter image description here

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  • $\begingroup$ Personally, I would forget the air effect. And think instead about how your body moves around on the scale. I could be wrong though, I'm no experimentalist, as I found out already on this site. $\endgroup$
    – user108787
    Commented Aug 4, 2016 at 1:18
  • $\begingroup$ Why forget the air effect? If I use a plastic plate I can get it to 3-4 grams, it must be the mass of air. $\endgroup$
    – Christian Pao.
    Commented Aug 4, 2016 at 1:25
  • $\begingroup$ OK, air could be part of it but if you google moving body on scale will it change my weight, they all say stay perfectly still or you won't get your true weight. So maybe bit of both. $\endgroup$
    – user108787
    Commented Aug 4, 2016 at 1:44

2 Answers 2

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There are 3 effects. First is the orientation of the balance. Moving around a floor that flexes or deforms, even slightly as you move your weight around, is a no-no. (similarly if the balance isn't isolated from you putting or removing weight from the table it is on). Second is that YOU carry an electric charge. So could a plastic plate. Does the reading persist? Or rapidly diminish? Charge will depend a lot on humidity. Different readings with same motion but different humidity suggests it is not air motion. Should be easy enough to dramatically change humidity in a room by boiling some water (if the room allows hotplates or is a kitchen...) placing pans of hot water near the balance should also reduce the charge your carry. Hmmm. I guess if you took an aluminum foil "hand" the size of yours, grounded it, and moved it in the same way as your hand, that you'd see the effect of wind alone. Air currents will generally diminish more rapidly than electrical charge, but the only way to tell for sure is to use a grounded conductor to move the air with.

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  • $\begingroup$ Nice careful analysis of possible experimental effects. $\endgroup$
    – Floris
    Commented Aug 4, 2016 at 1:47
  • $\begingroup$ I tried the experiment in different rooms with the same results, so I tend to exclude possible environmental factors. $\endgroup$
    – Christian Pao.
    Commented Aug 4, 2016 at 2:07
  • $\begingroup$ The scale tray (commonly plastic) could have static charge. The grounded hand would cause a force on the tray. $\endgroup$
    – Keith McClary
    Commented Aug 4, 2016 at 5:21
  • $\begingroup$ I let my hand at rest above the scale(without touching it) to be sure that no attraction was present. The -1 gram occurs only if I swing my hand fast as I described above. $\endgroup$
    – Christian Pao.
    Commented Aug 5, 2016 at 2:42
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If you detect a reading of -1 gram when you move your hand above the scale, it's quite possible you did generate some negative pressure.

This is a simple consequence of Benoulli's law, which states that

$$\frac12 \rho v^2 + P + \rho \cdot g\cdot h = \mathrm{const}$$

so when you increase the velocity $v$ of the air, you lower the pressure. Now you have to take into account the direction of the air flow as well: if you have air directed towards the scale, it has to change direction - that exchange of momentum results in a positive pressure on the scale (and pressure times area = force). But at the trailing edge of your hand moving over the scale, you have a vortex with horizontal velocity and therefore just lower pressure.

It's quite possibly a real effect - especially if you have carefully ruled out vibration etc.

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  • $\begingroup$ Thanks for giving a proper answer. So it's not all the cakes I'm eating, I knew it, I just knew it. :) $\endgroup$
    – user108787
    Commented Aug 4, 2016 at 1:56

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