5 ms·

Are you saying motion == acceleration?

aetherson 10y ago
He's making the point that relativity says that if something is "in motion" but never accelerates (including accelerating by turning), you can just put your point of reference with that object and define it as "at rest." In practice, in the real universe, everything is subject to acceleration. You can only have an object that is infinitely moving without accelerating in a toy universe with only one object. At which point it is probably more intuitive to define it as at rest instead, since there's nothing else around for it to be moving in relation to.
- 2_listerine_pls 10y ago
  I understand that everything is motionless (with respect to itself). However, we are talking about motion. So, it is obvious that there is an implicit frame of reference other than the object itself. It makes no sense for him to bring up that everything is motionless.
  - jacquesm 10y ago
    It makes perfect sense. Maybe not to you, but all motion is relative to some frame of reference. So if there is no acceleration in any dimension an object might as well be at rest until it collides with something (which would show either one or both objects to be in motion relative to a third reference frame). The universe does not have convenient 'true' grid lines along its axis to indicate the one true reference frame. You would be hard pressed to indicate a frame of reference that is motionless compared to everything else.
    - Dylan16807 10y ago
      The statement that motion is relative is true, but using it to argue that nothing has motion is what doesn't make sense. An object does not have to be alone in an entire universe to have no acceleration, or utterly negligible acceleration. And if it's not alone, it's trivial to have motion, even if you're not 100% sure which objects are moving.
      - jacquesm 10y ago
        > The statement that motion is relative is true Ok. > but using it to argue that nothing has motion is what doesn't make sense. Nothing has motion worth discussing that is not able to (a) change it's trajectory or (b) change it's configuration. As soon as those options are out you have an inert piece of matter, by definition not a perpetual motion machine. > An object does not have to be alone in an entire universe to have no acceleration, or utterly negligible acceleration. No, it can be without acceleration in an extremely crowded universe. And any acceleration that it does have will likely come from gravitational interaction with other bodies, or the impacts of particles or other objects (including light). > And if it's not alone, it's trivial to have motion, even if you're not 100% sure which objects are moving. That is where we depart. You are thinking of motion in a classical sense, possibly anchored in 'relative to the point of origin', but that's not how it works in physics where acceleration to a local frame of reference is the only meaningful one because it shows you whether or not the 'device' is the master of its own fate or merely along for the ride. That's a huge difference, roughly akin to 'dead' or 'alive' and of course there are always people who would like to split hairs over that but we all know what 'dead' means. In physics it can be a little harder to prove that an object really is dead but it gets a lot easier once you catch on to the inertial frame of reference idea. https://en.wikipedia.org/wiki/Inertial_frame_of_reference https://en.wikipedia.org/wiki/Inertial_frame_of_reference A final note that may drop the coin for you: An object 'at rest' does not change relative to it's own frame of reference, but an object that has a powersource can change relative to that same frame of reference. The reference frame stays still relative to where you found the object the first time and then you track it over time. If the change is just a translation then the object is dead, if it can change trajectory relative to that frame of reference, rotate about an axis faster or slower than how you found it initially or change it's configuration relative to the frame of reference (for instance, by operating some kind of actuator) then it is still 'alive'.
        Dylan16807 10y ago
        > As soon as those options are out you have an inert piece of matter, by definition not a perpetual motion machine. I think you're mixing up different comment threads. This was entirely about inert pieces of matter with 'dead' motion. > You are thinking of motion in a classical sens No I'm not. Look, if two billiard balls are 10 meters apart and that distance is growing by one meter per second, there is objectively motion there. It's inert motion, it's relative motion, but it's real motion. You can use any reference frame you want.

He's making the point that relativity says that if something is "in motion" but never accelerates (including accelerating by turning), you can just put your point of reference with that object and define it as "at rest." In practice, in the real universe, everything is subject to acceleration. You can only have an object that is infinitely moving without accelerating in a toy universe with only one object. At which point it is probably more intuitive to define it as at rest instead, since there's nothing else around for it to be moving in relation to.

2_listerine_pls 10y ago
I understand that everything is motionless (with respect to itself). However, we are talking about motion. So, it is obvious that there is an implicit frame of reference other than the object itself. It makes no sense for him to bring up that everything is motionless.
- jacquesm 10y ago
  It makes perfect sense. Maybe not to you, but all motion is relative to some frame of reference. So if there is no acceleration in any dimension an object might as well be at rest until it collides with something (which would show either one or both objects to be in motion relative to a third reference frame). The universe does not have convenient 'true' grid lines along its axis to indicate the one true reference frame. You would be hard pressed to indicate a frame of reference that is motionless compared to everything else.
  - Dylan16807 10y ago
    The statement that motion is relative is true, but using it to argue that nothing has motion is what doesn't make sense. An object does not have to be alone in an entire universe to have no acceleration, or utterly negligible acceleration. And if it's not alone, it's trivial to have motion, even if you're not 100% sure which objects are moving.
    - jacquesm 10y ago
      > The statement that motion is relative is true Ok. > but using it to argue that nothing has motion is what doesn't make sense. Nothing has motion worth discussing that is not able to (a) change it's trajectory or (b) change it's configuration. As soon as those options are out you have an inert piece of matter, by definition not a perpetual motion machine. > An object does not have to be alone in an entire universe to have no acceleration, or utterly negligible acceleration. No, it can be without acceleration in an extremely crowded universe. And any acceleration that it does have will likely come from gravitational interaction with other bodies, or the impacts of particles or other objects (including light). > And if it's not alone, it's trivial to have motion, even if you're not 100% sure which objects are moving. That is where we depart. You are thinking of motion in a classical sense, possibly anchored in 'relative to the point of origin', but that's not how it works in physics where acceleration to a local frame of reference is the only meaningful one because it shows you whether or not the 'device' is the master of its own fate or merely along for the ride. That's a huge difference, roughly akin to 'dead' or 'alive' and of course there are always people who would like to split hairs over that but we all know what 'dead' means. In physics it can be a little harder to prove that an object really is dead but it gets a lot easier once you catch on to the inertial frame of reference idea. https://en.wikipedia.org/wiki/Inertial_frame_of_reference https://en.wikipedia.org/wiki/Inertial_frame_of_reference A final note that may drop the coin for you: An object 'at rest' does not change relative to it's own frame of reference, but an object that has a powersource can change relative to that same frame of reference. The reference frame stays still relative to where you found the object the first time and then you track it over time. If the change is just a translation then the object is dead, if it can change trajectory relative to that frame of reference, rotate about an axis faster or slower than how you found it initially or change it's configuration relative to the frame of reference (for instance, by operating some kind of actuator) then it is still 'alive'.
      - Dylan16807 10y ago
        > As soon as those options are out you have an inert piece of matter, by definition not a perpetual motion machine. I think you're mixing up different comment threads. This was entirely about inert pieces of matter with 'dead' motion. > You are thinking of motion in a classical sens No I'm not. Look, if two billiard balls are 10 meters apart and that distance is growing by one meter per second, there is objectively motion there. It's inert motion, it's relative motion, but it's real motion. You can use any reference frame you want.