4 ms·

> If the reading of an accelerometer does change in freefall, then what it is measuring isn't the acceleration, necessarily, but the difference in acceleration

> If the reading of an accelerometer does change in freefall, then what it is measuring isn't the acceleration, necessarily, but the difference in acceleration between components in the device. This would come as a surprise to the designers. :) No, the acceleration on an object in free fall is in fact zero. It may be surprising, and may require rewriting your intuitions to fully grok, but this is indeed what GR says.

taeric 2y ago
Ah, fair. I was messing myself with the knowledge that you would be accelerating the whole fall. Easy to forget that we are always measuring proxies. (For example, speedometers aren't measuring how fast cars are moving.) Edit: (I'm actually curious what I said is fully wrong, btw? Accelerometers measure how much an inner piece drifts to other things based on differential in acceleration, no?) Edit2: I think you thought I was saying the accelerometer would not read zero in free fall? I meant more that you would continue accelerating, despite the reading being whatever it was.
- Filligree 2y ago
  > Edit: (I'm actually curious what I said is fully wrong, btw? Accelerometers measure how much an inner piece drifts to other things based on differential in acceleration, no?) In a steady state, all the components are accelerating equally. It measures the force between the components and uses that to compute acceleration, yeah, but the positional drift between the components is minuscule and only happens during jerk. (This describes one type of accelerometer. I'm not up to date on every possible or currently used design.)
  - taeric 2y ago
    Apologies, referring to the drift, I felt it was safe to assume you'd measure the force between the components. Not necessarily the distances. My point is that to an earth bound observer, something in free fall to the surface of the earth is accelerating faster to earth until they hit terminal velocity. This is despite the accelerometer on the falling body showing zero. Note that i don't think this adds any understanding to whether or not gravity is a force. But it does feel a lot like saying that, if you are in a river and able to stay still, that you aren't fighting the force of the river. Similarly, do we say that people standing on a moving sidewalk are stationary in the same way as someone standing on the sidewalk they are passing?

taeric 2y ago

Ah, fair. I was messing myself with the knowledge that you would be accelerating the whole fall. Easy to forget that we are always measuring proxies. (For example, speedometers aren't measuring how fast cars are moving.) Edit: (I'm actually curious what I said is fully wrong, btw? Accelerometers measure how much an inner piece drifts to other things based on differential in acceleration, no?) Edit2: I think you thought I was saying the accelerometer would not read zero in free fall? I meant more that you would continue accelerating, despite the reading being whatever it was.

Filligree 2y ago
> Edit: (I'm actually curious what I said is fully wrong, btw? Accelerometers measure how much an inner piece drifts to other things based on differential in acceleration, no?) In a steady state, all the components are accelerating equally. It measures the force between the components and uses that to compute acceleration, yeah, but the positional drift between the components is minuscule and only happens during jerk. (This describes one type of accelerometer. I'm not up to date on every possible or currently used design.)
- taeric 2y ago
  Apologies, referring to the drift, I felt it was safe to assume you'd measure the force between the components. Not necessarily the distances. My point is that to an earth bound observer, something in free fall to the surface of the earth is accelerating faster to earth until they hit terminal velocity. This is despite the accelerometer on the falling body showing zero. Note that i don't think this adds any understanding to whether or not gravity is a force. But it does feel a lot like saying that, if you are in a river and able to stay still, that you aren't fighting the force of the river. Similarly, do we say that people standing on a moving sidewalk are stationary in the same way as someone standing on the sidewalk they are passing?