3 ms·

I don't understand her comment. This is really easy to verify. You drop an iPhone, the gravity "acceleration" really goes to 0 - so far so good. But at rest

by tomp 2y ago

I don't understand her comment. This is really easy to verify. You drop an iPhone, the gravity "acceleration" really goes to 0 - so far so good. But at rest (i.e. holding the iPhone in your hand), the acceleration is pointing downwards, not upward as she claims.

Filligree 2y ago
Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.
recursive 2y ago
This shows that your accelerometer is broken. (or, more likely, that the vector is mislabeled) Try this thought experiment. Assume your phone is really accelerating downwards when at rest. Then let it enter a free fall i.e. drop it. Now it's accelerating down even more so the downwards acceleration should show an increase. Actually, it goes to zero. Relative to a free-fall, being stationary is an acceleration upwards.
dcrazy 2y ago
An iPhone held “at rest” cannot be accelerating downward, by definition.
- moralestapia 2y ago
  Force is not acceleration. That is extremely easy to understand but somehow you and all of these "eminent scientists" fail to grasp it.
  - Filligree 2y ago
    Force is not acceleration, and accelerometers do not measure force. We have other devices which do that, such as bathroom scales.
    - moralestapia 2y ago
      Yup. Paraphrasing, you should not measure gravity with an accelerometer, otherwise this thread happens.
      - Filligree 2y ago
        You can, but you should probably take three years of physics courses first.
AlexandrB 2y ago
https://old.reddit.com/r/AskElectronics/comments/2ufunr/why_is_my_atrest_accelerometer_showing_1g_with/ https://old.reddit.com/r/AskElectronics/comments/2ufunr/why_... I think there's some sign inversion happening there. But don't quote me, all the details are fuzzy to me.
warble 2y ago
To mimic the effect of gravity on the surface of the earth you could stand on a platform that is accelerating. As long as that platform is accelerating you will experience something similar to gravity on the surface of the earth. The relative direction of the acceleration would be "up". This is what she means.
- tomp 2y ago
  ok this clears it up, thanks! this is obviously a very confusing topic, as evidenced by the other replies that either don't get my post, or the post above :D
- deleted 2y ago
  [deleted]
taeric 2y ago
I think this is a distinction between push/pull? You are interpreting the reading as it being pulled down. You could also interpret it as you having to push it up. The example you give also doesn't make sense. The acceleration would not go to 0 when you drop something. The acceleration continues for the entire fall. It may be offset by friction if it hits terminal velocity. But, absent that, acceleration would continue the entire fall. 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. Which makes sense. Is like watching a balloon in a car when you start moving. (With the trick of whether the windows are open or not, of course.)
- Filligree 2y ago
  > 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.)

by tomp 2y ago

Filligree 2y ago

Your iPhone lies to you. The acceleration is indeed upwards, as you’ll see if you look at raw accelerometer data… or the feeling from your feet, which counts.

recursive 2y ago

This shows that your accelerometer is broken. (or, more likely, that the vector is mislabeled) Try this thought experiment. Assume your phone is really accelerating downwards when at rest. Then let it enter a free fall i.e. drop it. Now it's accelerating down even more so the downwards acceleration should show an increase. Actually, it goes to zero. Relative to a free-fall, being stationary is an acceleration upwards.

dcrazy 2y ago

An iPhone held “at rest” cannot be accelerating downward, by definition.

moralestapia 2y ago
Force is not acceleration. That is extremely easy to understand but somehow you and all of these "eminent scientists" fail to grasp it.
- Filligree 2y ago
  Force is not acceleration, and accelerometers do not measure force. We have other devices which do that, such as bathroom scales.
  - moralestapia 2y ago
    Yup. Paraphrasing, you should not measure gravity with an accelerometer, otherwise this thread happens.
    - Filligree 2y ago
      You can, but you should probably take three years of physics courses first.

AlexandrB 2y ago

https://old.reddit.com/r/AskElectronics/comments/2ufunr/why_is_my_atrest_accelerometer_showing_1g_with/ https://old.reddit.com/r/AskElectronics/comments/2ufunr/why_... I think there's some sign inversion happening there. But don't quote me, all the details are fuzzy to me.

warble 2y ago

To mimic the effect of gravity on the surface of the earth you could stand on a platform that is accelerating. As long as that platform is accelerating you will experience something similar to gravity on the surface of the earth. The relative direction of the acceleration would be "up". This is what she means.

tomp 2y ago
ok this clears it up, thanks! this is obviously a very confusing topic, as evidenced by the other replies that either don't get my post, or the post above :D
deleted 2y ago
[deleted]

taeric 2y ago

I think this is a distinction between push/pull? You are interpreting the reading as it being pulled down. You could also interpret it as you having to push it up. The example you give also doesn't make sense. The acceleration would not go to 0 when you drop something. The acceleration continues for the entire fall. It may be offset by friction if it hits terminal velocity. But, absent that, acceleration would continue the entire fall. 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. Which makes sense. Is like watching a balloon in a car when you start moving. (With the trick of whether the windows are open or not, of course.)

Filligree 2y ago
> 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.)