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View Full Version : A reference frame question

BigDon
2008-Apr-19, 08:50 PM

Reply number four by Blueshift gelled a question I've had but couldn't articulate until just now.

If the two different rotating spoke ends are in the same reference frame what would then constitue a seperate one? If they weren't physically connected?

Then I wondered what are the rules that define "seperate reference frame" and what would be the minimun discontinuity to qualify as one.

(I'm not doubting the concept, I just need some guidance)

cosmocrazy
2008-Apr-19, 08:59 PM
the term reference frame, is that meant in distance co-ordinates or time? and also from the pespective of the rotating spokes relative or from an observer relative to both spokes?

Steve Limpus
2008-Apr-20, 03:00 AM
Don't take this is as gospel... just want to see if I've got it right.

I've read that 'frame of reference' is just a fancy name for 'point of veiw'.

So I would say that for the ends of two different spokes in a wheel, their 'points of veiw' are similar, in that they are experiencing the same, or at least 'symmetric' (another word scientists like!), motion. Or in other words if you were to look at the motion of each spoke individually, you wouldn't be able to tell the difference.

The hub, on the other hand, has a different point of veiw, in that it's motion is materially different.

I saw this explained recently in the case for the circumference of a disc compared to the radius. A ruler along the circumference of the disc would be contracted, compared to a ruler along the diameter, such that the circumference would no longer be pi times the diameter. ( πd - did I get that right, first attempt at an equation!)

Somewhat correct??? :)

grant hutchison
2008-Apr-20, 06:04 PM
BigDon, it depends on what you mean by "reference frame". :)
We could say that the two points are in the same "rotating reference frame": then we'd have to invoke pseudoforces (centrifugal, Coriolis) to explain why things don't follow straightline trajectories when moving around inertially in that reference frame.
Or we could say that they're in different "instantaneous inertial reference frames": because they have velocities that point in different directions, and would tend to move apart if not acted on by the forces that hold the wheel together. In fact, the two points would be constantly changing their inertial reference frames as their velocity shifted during rotation.

Grant Hutchison

Jeff Root
2008-Apr-20, 08:52 PM
In any rotating object, there is an actual centripetal force, so the
idea of "invoking" a centrifugal pseudoforce seems superfluous.

-- Jeff, in Minneapolis

Jeff Root
2008-Apr-20, 09:02 PM
I think the first point that Don and anyone else asking this question
should get is that points on a rotating object are not in inertial motion,
so they are not in inertial frames.

However, the rotation can be analyzed into a large number of time slices
that are small enough that they can be considered instants, and at any
instant the motion of a point on a rotating object can be treated as
inertial. So you just string a large number of different inertial frames
together like frames in a movie film, to simulate rotational motion.

And I don't know whether that constitutes a pun or not.

-- Jeff, in Minneapolis

grant hutchison
2008-Apr-20, 09:30 PM
In any rotating object, there is an actual centripetal force, so the
idea of "invoking" a centrifugal pseudoforce seems superfluous.In a rotating frame, held together by centripetal forces, an inertial object fails to travel in straight lines. The inhabitants of the rotating frame need to invoke a pseudoforce to account for that. If they say, "Ah, we and the 'stationary' objects around us are evidently rotating as a result of some centripetal acceleration," they have mentally stepped outside their rotating reference frame, and have become part of the external inertial frame.

Grant Hutchison