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mani Dec3-04 12:47 AM

Reference Frame
Under "Dynamics", an Inertial frame of Reference can be defined absolutely as one that does not use "pseudo" or inertial forces.
Under "Kinematics" how do we define absolutely (any) "Frame of Referaece"?

mani Dec4-04 06:30 AM

"Reference Frame"
How is a "Reference Frame" defined absolutely (i.e. without introducing another Frame) under Kinematics ?

ZapperZ Dec4-04 06:44 AM


Quote by mani
How is a "Reference Frame" defined absolutely (i.e. without introducing another Frame) under Kinematics ?

Er... the moment you say "I'm going to measure the position of everything with respect to this point here", then you have defined a reference frame. An "inertial reference frame" will have a more stringent constraint than that.


cepheid Dec5-04 12:45 AM

David J. Griffiths offers some insights in his Introduction to Electrodynamics


Page 477:

Classical Mechanics obeys the principle of relativity: the same laws apply in any inertial reference frame. By "inertial", I mean that the system is at rest or moving with constant velocity. 1

1. This raises an awkward problem. If the laws of physics hold just as well in a uniformly moving frame, then we have no way of identifying the "rest" frame in the first place, and hence no way of checking that some other frame is moving at constant velocity. To avoid this trap, we define an inertial frame formally as one in which Newton's first law holds. If you want to know whether you're in an inertial frame, throw some rocks around -- if they move in straight lines at a constant speed, you've got yourself an inertial frame, and any frame moving at constant velocity with respect to you will be another inertial frame.

I don't see how it makes a difference whether you are working with kinematics or dynamics; both are just aspects of classical mechanics, in which this definition applies.

cepheid Dec5-04 01:11 AM

Okay...that's really not cool! Don't double post man! :mad: I posted a reply here:

then I see a duplicate thread with the exact same question.

mani Dec6-04 12:58 AM

"I don't see how it makes a difference whether you are working with kinematics or dynamics;"

Kinematics is "Space" (or Geometry) + Time; where we can talk about moving "points"only.
Inertia is a concept that comes with matter under Dynamics.
Rotation can be detected only with inertia.

rayjohn01 Dec7-04 03:50 PM

As I understand it a 'frame of reference' is not dependant on the system considered per se, it is a set of values given to any and all variables within the system at some initial condition , and hence a referential situation.
To me dynamic and kinematic are the same thing involving motion , they may or may not involve rotation , which is a dimension , not included in inertial frames .
You must define carefully which variables you include or not -- then apply values to them as a reference 'point' ( multidimensional ).
To reply to whoever -- rotation has nothing to do with inertia -- inertia in the rotational sense is clearly due to momentum but momentum is the resistance to change in whatever direction in space. Rotation of zero mass does not involve momentum , it could be just the frame of reference ..

mani Dec14-04 07:15 AM

That was a bit above my head!
In kinematics, i can take a "rigid" set of points as a Frame, if someone can assure me that it is not rotating but moving only translation. If any other set of points is moving only in translation w.r.t. the above "certified" frame, that also can be accepted as a frame.
Absolute rotation of only a material frame can be detected by the inertial forces

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