# Rest frames

1. Feb 25, 2010

### wam_mi

Hi there,

Could anyone explain to me what is the different between the two terms

(i) Original rest frame
(ii) Instantaneous rest frame

When do we prefer to work in the original rest frame but not the instantaneous one and why? And when do we prefer to work in the instantaneous rest frame but not the original one ,and why? Could anyone explain this perhaps with an example?

Thanks a lot!

2. Feb 25, 2010

### A.T.

I guess you mean:

(i) Original inertial rest frame of an accelerating object
(ii) Instantaneous inertial rest frame of an accelerating object

(i) is just (ii) at the begin of the observation.

3. Feb 25, 2010

### sylas

These are not really technical terms with a single well defined meanings, but merely ways to identify a rest frame in a particular problem or example. You might say the rest frame of the launch pad, or the rest frame of the Klingon, or the rest frame "A", or what ever.

The terms you are using are most likely with reference to a problem in which there is some kind of accelerated motion; a space ship, perhaps. A space ship which accelerates does not correspond to an inertial frame. But you speak of various aspects in a given rest frame. So which frame could you take?

An "initial" rest frame would be a useful way to refer to the rest frame of a coasting spaceship before it starts some kind of accelerated motion.

An "instantaneous" rest frame would the the frame in which the velocity of the ship is zero at that instant -- even though it is accelerating and changing velocity -- and (usually) in which the location of the spaceship is the origin of the frame.

What frame you use depends on what you want to do.

If you want to calculate, for example, the angular size of a star that the ship is approaching, then you could use the instantaneous rest frame. But if you want to calculate the total age of the pilot, it may be easier to pick the initial rest frame, and just integrate the proper time in that frame. What frame you use and what you call it depends on the problem you are considering.

Cheers -- sylas