Length of Moving Rod Relative to Frame S

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Discussion Overview

The discussion revolves around the measurement of a moving rod's length in different frames of reference, particularly focusing on relativistic effects and the behavior of light emitted from the rod. Participants explore concepts related to length contraction, the perception of perpendicularity of light, and the implications of relative motion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the rod will measure 1 m in length for the observer moving with it, as it is at rest relative to that observer.
  • Questions arise regarding the behavior of light emitted perpendicularly from the rod and whether it remains perpendicular upon reaching a moving spaceship.
  • There is uncertainty about the definitions of "perpendicular" in relation to the rod and the spaceship, as well as the conditions of their motion (constant velocity vs. acceleration).
  • Some participants clarify that if the rod and spaceship are at rest relative to each other, the direction of the spaceship's motion does not affect the measurement of light's path.
  • Concerns are raised about the relevance of a linked discussion, with participants noting that it addresses a different scenario involving a mirror rather than the current topic of measuring a rod.

Areas of Agreement / Disagreement

Participants generally agree that the rod's length is measured as 1 m by the observer carrying it. However, there is disagreement and uncertainty regarding the implications of light emitted from the rod and how it interacts with a moving spaceship, as well as the interpretation of perpendicularity in this context.

Contextual Notes

Participants express confusion about the specific conditions of motion and the definitions used in the discussion, indicating a need for clarity on the assumptions made regarding the scenarios being analyzed.

Snip3r
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Consider a rod of length 1 m and I are in frame S. Now i move at a velocity considerable to velocity of light wrt S. I take the rod with me (with its length parallel to motion). Now when i measure the length of the rod what will it be?i think it will still be 1 m for me but just wanted to confirm
 
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Yes, since the rod remains at rest with respect to you, you'll measure it to have its rest length of 1 m.
 
Hi Doc Al

--------------------------------------------------------------------------------
Yes, since the rod remains at rest with respect to you, you'll measure it to have its rest length of 1 m.

If you shoot light out from the rod in a perpendicular direction to the spaceship does it remain perpendicular when it hits the space ship? Does the distance matter?
 
Qzit said:
If you shoot light out from the rod in a perpendicular direction to the spaceship does it remain perpendicular when it hits the space ship?
As seen by observers in the spaceship, the light path is perpendicular to the rod and the ship.
Does the distance matter?
What distance?
 
If you shoot light out from the rod in a perpendicular direction to the spaceship does it remain perpendicular when it hits the space ship? Does the distance matter?

Doc Al...how did you ever interpret this??
Is the 'perpendicular' to the rod or the spaceship or both? What direction is the spaceship moving relative to the rod? Is it accelerating or moving with a constant velocity [fixed speed and direction]?

As seen by observers in the spaceship, the light path is perpendicular to the rod and the ship.

if the ship is moving at constant velocity parallel to the rod, right??
 
I can't speak for Doc Al, but I assumed that the "space ship" Qzit was referring to was the one on which the Snip3r was carrying the rod. Since they are all (Snip3r, the rod, and the spaceship) are all motionless with respect to each other, it really does not matter whether is arranged in the direction the spaceship is moving.
 
HallsofIvy...oh good grief, no wonder...I did not think of that scenario...
Thank you!

That also answers my post #7...
and to all a good night!

But that thread relates...a different perspective...so I'll leave it...
 
HallsofIvy said:
I can't speak for Doc Al, but I assumed that the "space ship" Qzit was referring to was the one on which the Snip3r was carrying the rod.
Exactly. (If Qzit wants to discuss a different scenario, let him define it.)
 
  • #10
Naty1 said:
The conclusion here seems different...is it?? Now I'm not sure...time to think a bit more!

https://www.physicsforums.com/showthread.php?t=562144
The discussion in that link doesn't seem to have anything to do with this topic. There, you are moving at high speed relative to a mirror and shining as light at it, here, you are measuring a stick you are carrying with you.
 
  • #11
The discussion in that link doesn't seem to have anything to do with this topic. There, you are moving at high speed relative to a mirror and shining as light at it, here, you are measuring a stick you are carrying with you.

i agree... that's why I said a "different perspective" above ...maybe I should have said different frames of reference or different situation.
 

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