4 ms·

That's just because gravity is also accelerating the accelerometer, and the accelerometer can only measure acceleration relative to the body of the instrument.

by ars 2y ago

That's just because gravity is also accelerating the accelerometer, and the accelerometer can only measure acceleration relative to the body of the instrument. I've heard this argument: Because gravity is the only force that acts on all objects, including the measuring object, this makes it not a force. This is not actually an explanation, it's more of a definition, and not a useful one.

Filligree 2y ago
If you attach an accelerometer to a rocket and turn on the rocket, it’ll measure acceleration even in deep space. This is despite all parts of the accelerometer accelerating equally. In fact it measures absolute acceleration, not relative.
- tzs 2y ago
  A rocket would not accelerate all parts of the accelerometer equally.
  - Filligree 2y ago
    Not in the first couple of seconds, but by the time everything settles down it certainly will.
    - deleted 2y ago
      [deleted]
    - cryptonector 2y ago
      Sorry, but no. As long as the rocket engine is producing thrust then the entire rocket will only feel the normal forces caused by that engine because those forces are electromagnetic in nature (electron clouds pushing against electron clouds). It's only because the rocket will be made of rigid materials (but still, made of atomic matter) that the forces are eventually communicated to the nose of the ship and everything in the ship. That "eventually" happens to be very fast because the speed of light is very fast, but it is not instantaneous throughout the ship, and that is why you feel the acceleration due to the engine's thrust. Whereas when the engine is off the ship free-falls, and in the free-falling case any gravitational forces will have the same effect on every part of the ship at the same time -instantaneously-. Now if the ship is large enough then the gravitational field(s) it traverses won't be uniform enough, and then the ship will feel stresses ("tidal forces") as a result.
- cryptonector 2y ago
  As u/tzs explains, a rocket engine does not cause a uniform field that accelerates the rocket. It only creates a force at the engine's combustion chamber and bell nozzle. That force is then communicated to the rest of the ship via gazillions of interactions between the electrons and protons making up the atoms that make up the ship and its contents. In fact, even at the engine itself it's the same story. It is these electromagnetic interactions that we call the "normal force" that we feel and that our accelerometers measure. It is too trivial to make the mistake you did, and then to conclude that gravity is not a force. But that's not really a correct view. A better view is that there are at least two equivalent understandings of gravity: one where it curves spacetime, and the other where it causes accelerations (therefore is a force) that cause photons and massive particles to follow the paths that the other interpretation would have them follow due to spacetime curvature.

by ars 2y ago

Filligree 2y ago

If you attach an accelerometer to a rocket and turn on the rocket, it’ll measure acceleration even in deep space. This is despite all parts of the accelerometer accelerating equally. In fact it measures absolute acceleration, not relative.

tzs 2y ago
A rocket would not accelerate all parts of the accelerometer equally.
- Filligree 2y ago
  Not in the first couple of seconds, but by the time everything settles down it certainly will.
  - deleted 2y ago
    [deleted]
  - cryptonector 2y ago
    Sorry, but no. As long as the rocket engine is producing thrust then the entire rocket will only feel the normal forces caused by that engine because those forces are electromagnetic in nature (electron clouds pushing against electron clouds). It's only because the rocket will be made of rigid materials (but still, made of atomic matter) that the forces are eventually communicated to the nose of the ship and everything in the ship. That "eventually" happens to be very fast because the speed of light is very fast, but it is not instantaneous throughout the ship, and that is why you feel the acceleration due to the engine's thrust. Whereas when the engine is off the ship free-falls, and in the free-falling case any gravitational forces will have the same effect on every part of the ship at the same time -instantaneously-. Now if the ship is large enough then the gravitational field(s) it traverses won't be uniform enough, and then the ship will feel stresses ("tidal forces") as a result.
cryptonector 2y ago
As u/tzs explains, a rocket engine does not cause a uniform field that accelerates the rocket. It only creates a force at the engine's combustion chamber and bell nozzle. That force is then communicated to the rest of the ship via gazillions of interactions between the electrons and protons making up the atoms that make up the ship and its contents. In fact, even at the engine itself it's the same story. It is these electromagnetic interactions that we call the "normal force" that we feel and that our accelerometers measure. It is too trivial to make the mistake you did, and then to conclude that gravity is not a force. But that's not really a correct view. A better view is that there are at least two equivalent understandings of gravity: one where it curves spacetime, and the other where it causes accelerations (therefore is a force) that cause photons and massive particles to follow the paths that the other interpretation would have them follow due to spacetime curvature.