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I am drafting a patent application where one part of the invention measures a force. I'd like to distinguish this from the prior art, which measures a displacement. But for all the force measurement methods I know about, there's some kind of motion or deformation or reconfiguration going on. (For example, a piezo sensor produces a signal proportional to force, but there is deformation going on in the crystal.)

Is there any way to measure change in force without motion or deformation? (Perhaps I can distinguish the displacement sensors as "displacement without force" -- after all, two things can be in different places, without some particular force required to keep them apart.)

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  • $\begingroup$ Are you asking for situations in which there is no actual motion/deformation, or no potential motion/deformation. In the former case there is the beam balance or current balance. The potential is there, but the measurement is made when there is no motion/deformation. $\endgroup$ – sammy gerbil Aug 1 '17 at 18:17
  • $\begingroup$ If you draft a patent application for your invention, and you know that it works, then I don't think you have to ask us. Anyways, I believe it's off topic here. $\endgroup$ – Wrichik Basu Aug 1 '17 at 18:20
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    $\begingroup$ MEMS accelerometers (such as air bag sensors) work by detecting very small displacements capacitively and applying a counter force to keep the sensor motionless. The acceleration is then derived from the voltage needed to keep the sensor part steady. So, yes, very small movement is needed, but it is rapidly corrected for. $\endgroup$ – Jon Custer Aug 1 '17 at 18:39
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    $\begingroup$ There are no "perfectly rigid" materials in real life. Any form of force will create a stress and therefore produce some motion. In practice of course the motion may be so small that you can ignore it - but there are many well known force transducers devices that already use that fact, so you probably need something more specific to claim a patent on a new device. $\endgroup$ – alephzero Aug 1 '17 at 18:46
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    $\begingroup$ The question does seem a bit odd. If your invention indeed measures force without displacement, then there is a method of measuring force without displacement and you didn't need to ask the question. If your invention measures force via. displacement, then your description for the patent is incorrect regardless of the question. $\endgroup$ – Cort Ammon Aug 1 '17 at 19:05
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Deformation (by which I mean changes in relative positions or velocities of a system's constituents) is part of the very meaning of the force concept. Indeed, when acceleration occurs without any accompanying deformation, we are inclined to assume that the acceleration is a manifestation of the choice of a non-inertial reference frame rather than a force. That is precisely the reasoning behind the modern view that gravity should not be thought of as a force.

So it all depends on whether you are trying to eliminate distortion for all practical purposes, or trying to do so as a matter of principle. I would argue that the latter is impossible, but the former is perfectly doable.

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