I’ve never used them before, but apparently piezoresistive accelerometers use one or more resistors to sense acceleration. (I’ve only used piezoelectric accelerometers, which use a crystal.) The resistor(s) are configured as strain gauges and used in a Whetstone Bridge. Most importantly I have read this type of accelerometer can measure “DC” (constant) acceleration.
I am trying to understand what the response of the accelerometer would be in certain circumstances. So dumb questions time:
1. If the accelerometer is simply setting on a table, will it read 9.8 ms[sup]-2[/sup]? I have heard it will. If this is true, it is a bit odd, in that the accelerometer is not actually accelerating at all.
2. If I flip the accelerometer over and set it back on the table, will it read -9.8 ms[sup]-2[/sup]? Again, I have heard it will.
3. If the accelerometer is in free fall, will it read 9.8 ms[sup]-2[/sup]? I recall an old post on the SDMB where Chronos said it would read 0 ms[sup]-2[/sup] in free fall. Which is a bit odd, in that the accelerometer is accelerating at 9.8 ms[sup]-2[/sup] in free fall.
4. I put the accelerometer in a centrifuge, and orient it so that it reads acceleration in the radial direction. If the centrifuge is spinning at a constant RPM, will the accelerometer read v[sup]2[/sup]/r?
So in a nutshell, I am trying to understand how these things work from a usage point-of-view. It appears this type of accelerometer does not measure actual acceleration, but only F/m, where F is one of the forces on the accelerometer. Or perhaps this is all about the frame of reference. I dunno…
You have a mini version of the classic person in a lift. Force due to gravity and due to acceleration are indistinguishable. The little force bridge in the accelerometer is a mass on a spring. It does indeed only measure force. It has no external reference, so all forces are relative to its reference frame.
It will read 0 ms[sup]-2[/sup] while falling in a vacuum. In the atmosphere, the acceleration that it reads will increase until it reaches its terminal velocity. Then it will read 9.8 ms[sup]-2[/sup].
It’s not odd at all that it reads 9.8 m/s^2 when sitting on a table, because that’s exactly the acceleration it has then. Likewise, it’s not at all odd that it reads zero when in free fall, because that’s exactly the acceleration it has then. Gravity is exactly the same sort of force as centrifugal force or any of those other forces you were taught were “fictitious”, in that it only exists as a “force” as an artifact of the non-inertial reference frame we choose to use.
What is rather counterintuitive is that even though we’re accelerating at 9.8 m/s^2 up, and the point opposite us on the planet’s surface has that acceleration in the opposite direction, we’re still remaining at rest relative to each other… but that’s where curvature of spacetime comes in.
Thanks. And am still trying to wrap my head around this.
If the accelerometer is inside a thrown baseball and is measuring acceleration in the horizontal axis, will it read the deceleration when it is caught in a mitt?
Accelerometers really are “deviation from free-fall-meters” - they report a non-zero output only when the device is doing something different from what an object in free fall would do, and they report exactly zero when they are doing what an object in free fall would do.
No reason it shouldn’t. As a practical matter, that acceleration might be too large for it to register correctly, but an ideal instrument certainly would.
Yes. And vertical as well. An interesting (to me) point is that it will read zero (assuming vacuum) from the instant it leaves the throwing hand. (or the bat) It is very common for people (even those with physics degrees) to think that free-fall only starts at the top of the arc. This comes up in discussions about the “vomit comet” aircraft that astronauts train in.