7 ms·

> ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointi

> ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointing downwards.

m4r35n357 5y ago
_You_ are providing that acceleration. You need to drop it!
cygx 5y ago
Assuming you're standing on the surface of the earth, that's wrong: You're being accelerated upwards, not downwards. Cf the introductory paragraph and "Physical principles" section of https://en.wikipedia.org/wiki/Accelerometer https://en.wikipedia.org/wiki/Accelerometer
- dilippkumar 5y ago
  Sorry, I interpreted the - sign in the opposite direction. Either ways, the accelerometer is registering something. If gravity was purely a ‘pseudo force’ as GGP said, I should’ve expected a 0 reading. Am I misunderstanding the point made about the accelerometer?
  - cygx 5y ago
    You're at rest relative to Earth's surface. From the perspective of that particular frame, the gravitational (pseudo-)force gets balanced by the normal force. But the gravitational force is 'actually' zero, leaving the normal force accelerating you upwards to be measured by your accelerometer.
    - zackees 5y ago
      The gravitational force is zero, but it balances out with a real force which is not zero?
    - deleted 5y ago
      [deleted]
  - roywiggins 5y ago
    The feeling you have of force is due to resting on something that's holding you up. If you take that away, you would be in the same gravitational field but feel no forces on you and your accelerometer would measure 0m/s/s. If you jump off a bridge, during your brief flight you could look at your accelerometer and see zero. The gravitational influence on you hasn't changed at all by jumping off a bridge- the only change was removing the upward force that stopped you from falling.
    - o-__-o 5y ago
      Or record your screen and drop your phone from a second floor balcony to see what the accelerometer reads
- kazinator 5y ago
  If you're standing, you're not accelerating anywhere. Acceleration is a change in velocity, which is a change in position. The forces on you are balanced: the force of gravity is balanced by the force exerted upon you by the ground. You're not going anywhere, and therefore you're not accelerating. An accelerometer works by comparing the difference between some mass that is acted upon by an external force and a related mass that is loosely connected to it (e.g. by a spring-like linkage. whose strain we can measure) as not to be acted upon by that force directly. If you move the casing of an accelerometer, the sprung mass inside it lags behind, and thereby the acceleration can be measured via strain gauges or whatever. Gravity defies accelerometers because, unsurprisingly, gravity acts on all of the the masses contained in an object equally. (At least any gravitational field you are likely to encounter in ordinary experience is going be almost perfectly uniform across an everyday object.) It is when you are in free-fall, that is when you're accelerating due to gravity. And that's exactly when the accelerometer doesn't tell you anything, because every part of the object is in the same free-fall; all of you is accelerating. The accelerometer is differential. It doesn't see a difference, so it doesn't see acceleration. You now that free-fall is acceleration because orbiting satellites are in free fall and they follow circular paths. Objects will not move in a circle unless a force accelerates them toward the center. Like when you spin a mass on the end of as string, it goes in a circle due to the centripetal force exerted by the tension carried in the string. When you're standing on the planet, your accelerometer is lying to you. Gravity is acting on the sprung mass inside the accelerometer, and it's also acting on the case which holds that mass. But the case is constrained from moving, whereas the sprung mass isn't. So a differential strain develops, which "looks and feels" like acceleration. In summary, an accelerometer is an instrument which measures the acceleration due to a force which is applied just to its casing, and not to some critical piece of mass held inside. Gravity causes equal acceleration of the casing and that piece of mass, and so gravitational acceleration is immeasurable by that accelerometer. Acceleration can be measured with regard to some frame of reference via distance and time calculations.
chronial 5y ago
You misread. Your phone is accelerating upwards at 9.8m/s^2. That is also what it displays. If you have an accelerometer that displays a history (I recommend the app phyphox), you will also see that acceleration while your phone is falling.
- wrycoder 5y ago
  The phone will measure zero acceleration while it's falling.
  - zackees 5y ago
    If we replace gravity with magnetism can we make the same conclusion? How about the forces experience from title forces. The difference in gravitational pull on the moon (tidal forces) caused the moon to be locked in orbit with the earth. If gravity isn’t a force, what stopped the moon from rotating faster than its orbit?
  - whoopdedo 5y ago
    Any skydivers wish to confirm this? But the wind would push on the phone. So any lunar skydivers wish to confirm?
    - DarmokJalad1701 5y ago
      A better analogy is something that is in a stable orbit.
  - chronial 5y ago
    Apparently I forgot to type the "go away" that was supposed to go between the "acceleration" and "while" in that sentence ^^.
jon_richards 5y ago
That’s because you aren’t in free fall.
- ctdonath 5y ago
  If I step off a bridge, why does the distance between me and the very large nearby spherical mass begin decreasing at an, umm, accelerating rate?
  - babesh 5y ago
    The surface is accelerating up at you relative to the gravitational field.
    - mrow84 5y ago
      What happens if two people jump off bridges on opposite sides of the earth? How does that work in that interpretation?
      - 317070 5y ago
        Same thing? For each of them, they see themselves non-accelerating with respect to the gravitational field. But they do see the soil accelerating towards them against the gravitational field. Why is the soil not in free-fall? Because it feels normal force of all the earth below. And why is not the whole earth in free fall towards its core? Because it has an electrostatic force (a real one!) which keeps all the atoms and molecules from collapsing on itself. If it would collapse on itself, it would first become a neutron star and then a black hole.
        mrow84 5y ago
        It still isn’t obvious to me how, if “the surface is accelerating up at you relative to the gravitational field” on both sides of the earth, the earth isn’t pulling itself apart. To be clear, my confusion is about how this interpretation is consistent with the facts, not the facts themselves (I think).
DarmokJalad1701 5y ago
If you were in free-fall, (preferably in a stable orbit and not falling off a tall building) and checked your accelerometer, you would read zero. Even though you are still being accelerated by gravity.

> ...Being accelerated by such a pseudo-force won't register on an accelerometer... Just checked the accelerometer on my phone. It reads around 9.8m/s/s pointing downwards.

m4r35n357 5y ago

_You_ are providing that acceleration. You need to drop it!

cygx 5y ago

Assuming you're standing on the surface of the earth, that's wrong: You're being accelerated upwards, not downwards. Cf the introductory paragraph and "Physical principles" section of https://en.wikipedia.org/wiki/Accelerometer https://en.wikipedia.org/wiki/Accelerometer

dilippkumar 5y ago
Sorry, I interpreted the - sign in the opposite direction. Either ways, the accelerometer is registering something. If gravity was purely a ‘pseudo force’ as GGP said, I should’ve expected a 0 reading. Am I misunderstanding the point made about the accelerometer?
- cygx 5y ago
  You're at rest relative to Earth's surface. From the perspective of that particular frame, the gravitational (pseudo-)force gets balanced by the normal force. But the gravitational force is 'actually' zero, leaving the normal force accelerating you upwards to be measured by your accelerometer.
  - zackees 5y ago
    The gravitational force is zero, but it balances out with a real force which is not zero?
  - deleted 5y ago
    [deleted]
- roywiggins 5y ago
  The feeling you have of force is due to resting on something that's holding you up. If you take that away, you would be in the same gravitational field but feel no forces on you and your accelerometer would measure 0m/s/s. If you jump off a bridge, during your brief flight you could look at your accelerometer and see zero. The gravitational influence on you hasn't changed at all by jumping off a bridge- the only change was removing the upward force that stopped you from falling.
  - o-__-o 5y ago
    Or record your screen and drop your phone from a second floor balcony to see what the accelerometer reads
kazinator 5y ago
If you're standing, you're not accelerating anywhere. Acceleration is a change in velocity, which is a change in position. The forces on you are balanced: the force of gravity is balanced by the force exerted upon you by the ground. You're not going anywhere, and therefore you're not accelerating. An accelerometer works by comparing the difference between some mass that is acted upon by an external force and a related mass that is loosely connected to it (e.g. by a spring-like linkage. whose strain we can measure) as not to be acted upon by that force directly. If you move the casing of an accelerometer, the sprung mass inside it lags behind, and thereby the acceleration can be measured via strain gauges or whatever. Gravity defies accelerometers because, unsurprisingly, gravity acts on all of the the masses contained in an object equally. (At least any gravitational field you are likely to encounter in ordinary experience is going be almost perfectly uniform across an everyday object.) It is when you are in free-fall, that is when you're accelerating due to gravity. And that's exactly when the accelerometer doesn't tell you anything, because every part of the object is in the same free-fall; all of you is accelerating. The accelerometer is differential. It doesn't see a difference, so it doesn't see acceleration. You now that free-fall is acceleration because orbiting satellites are in free fall and they follow circular paths. Objects will not move in a circle unless a force accelerates them toward the center. Like when you spin a mass on the end of as string, it goes in a circle due to the centripetal force exerted by the tension carried in the string. When you're standing on the planet, your accelerometer is lying to you. Gravity is acting on the sprung mass inside the accelerometer, and it's also acting on the case which holds that mass. But the case is constrained from moving, whereas the sprung mass isn't. So a differential strain develops, which "looks and feels" like acceleration. In summary, an accelerometer is an instrument which measures the acceleration due to a force which is applied just to its casing, and not to some critical piece of mass held inside. Gravity causes equal acceleration of the casing and that piece of mass, and so gravitational acceleration is immeasurable by that accelerometer. Acceleration can be measured with regard to some frame of reference via distance and time calculations.

chronial 5y ago

You misread. Your phone is accelerating upwards at 9.8m/s^2. That is also what it displays. If you have an accelerometer that displays a history (I recommend the app phyphox), you will also see that acceleration while your phone is falling.

wrycoder 5y ago
The phone will measure zero acceleration while it's falling.
- zackees 5y ago
  If we replace gravity with magnetism can we make the same conclusion? How about the forces experience from title forces. The difference in gravitational pull on the moon (tidal forces) caused the moon to be locked in orbit with the earth. If gravity isn’t a force, what stopped the moon from rotating faster than its orbit?
- whoopdedo 5y ago
  Any skydivers wish to confirm this? But the wind would push on the phone. So any lunar skydivers wish to confirm?
  - DarmokJalad1701 5y ago
    A better analogy is something that is in a stable orbit.
- chronial 5y ago
  Apparently I forgot to type the "go away" that was supposed to go between the "acceleration" and "while" in that sentence ^^.

jon_richards 5y ago

That’s because you aren’t in free fall.

ctdonath 5y ago
If I step off a bridge, why does the distance between me and the very large nearby spherical mass begin decreasing at an, umm, accelerating rate?
- babesh 5y ago
  The surface is accelerating up at you relative to the gravitational field.
  - mrow84 5y ago
    What happens if two people jump off bridges on opposite sides of the earth? How does that work in that interpretation?
    - 317070 5y ago
      Same thing? For each of them, they see themselves non-accelerating with respect to the gravitational field. But they do see the soil accelerating towards them against the gravitational field. Why is the soil not in free-fall? Because it feels normal force of all the earth below. And why is not the whole earth in free fall towards its core? Because it has an electrostatic force (a real one!) which keeps all the atoms and molecules from collapsing on itself. If it would collapse on itself, it would first become a neutron star and then a black hole.
      - mrow84 5y ago
        It still isn’t obvious to me how, if “the surface is accelerating up at you relative to the gravitational field” on both sides of the earth, the earth isn’t pulling itself apart. To be clear, my confusion is about how this interpretation is consistent with the facts, not the facts themselves (I think).

DarmokJalad1701 5y ago

If you were in free-fall, (preferably in a stable orbit and not falling off a tall building) and checked your accelerometer, you would read zero. Even though you are still being accelerated by gravity.