How does the speed of light affect the rotation of a disc?

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Discussion Overview

The discussion revolves around the implications of the speed of light on the behavior of very long objects, such as a rod or a billiards table, when one end is moved. Participants explore concepts from relativity, rigidity, and mechanical wave propagation, considering both theoretical and hypothetical scenarios.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that if a very long rod is pulled, the other end will not move instantaneously due to the finite speed of signal transmission, which is limited by the speed of sound in the material of the rod.
  • Others argue that relativity prohibits the existence of perfectly rigid objects, meaning that any movement will take time to propagate through the rod.
  • A participant suggests that the speed of mechanical pulses through a rod or billiard balls depends on the molecular structure of the material, with a question raised about which material would allow the fastest pulse.
  • Some participants discuss the analogy of a billiards table, questioning how long it would take for the last ball to move after the first is struck, emphasizing that the response time is not instantaneous.
  • There are hypothetical scenarios presented, such as a one light-year-long staff, questioning how movement would be perceived from a distance and whether it could exceed the speed of light.
  • One participant mentions that the speed of sound in materials like diamond is significantly slower than the speed of light, reinforcing the idea that no mechanical signal can travel faster than light.

Areas of Agreement / Disagreement

Participants generally agree that no object can be perfectly rigid and that movement will not be instantaneous. However, there are competing views regarding the specifics of how mechanical waves propagate and the implications of hypothetical scenarios involving very long objects.

Contextual Notes

Limitations include assumptions about the rigidity of materials, the nature of mechanical wave propagation, and the hypothetical scenarios presented, which may not account for practical constraints or real-world physics.

zarbanx
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according to the theory nothing is faster than light so if there were a rod very very long(eigth ligth min long) and then we were to pull it across with a velocity(applying either a force or impulse) then what will happen to the length of the rod?

will it increase becoz the other end of the rod will start moving only after eigth minutes...
 
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zarbanx said:
.. will it increase becoz the other end of the rod will start moving only after eigth minutes...

Yes. Relativity requires nothing is infinitely rigid (unstretchable).
 
Impulses travel through a rod at the speed of sound in the rod.
 
What if the rod were infinitely rigid?
 
Thoreau said:
What if the rod were infinitely rigid?

Relativity prohibits objects to be infinitely rigid (as already posted above).

Read the thread completely next time.
 
Of course. Sorry.
 
yes nothing is rigid bt nothing is elastic to tht extent the rod is bound to break...isnt it
 
that was what i had in mind. but then it doesn't necessarily have to. it depends on the distance the rod is moved and the velocity of the displacement.
 
Crazy question but I have to ask:

What if instead of a rod you had a very long (eight light minutes long) billiards table and lined up the balls touching each other in a straight line with guides and struck the first ball with the cue ball. How long would it be until the ball at the other end jumped off?
 
  • #10
ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move...
crazy isn't it
 
  • #11
zarbanx said:
ur question is similar to mine and the answer to ur ques is the answer to mine

"NOTHING IS PERFECTLY RIGID" and thus only after eigth minutes will the ball at the other end move...
crazy isn't it
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing? :rolleyes:
 
  • #12
DaveC426913 said:
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light:

If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?
 
  • #13
DaveC426913 said:
It will take much, much longer than eight minutes, since the pulse will travel at the speed of the billiard balls, not the speed of light.

Nice thread title, BTW. You in bulk marketing? :rolleyes:


bulk marketing why do u think that??
 
  • #14
bulk marketing why do u think that??
Because your post sounds like spam.
 
  • #15
peter0302 said:
Because your post sounds like spam.

i don't get it dear and moreover i asked for an answer frm dave not u peter
 
  • #16
zarbanx said:
i don't get it dear and moreover i asked for an answer frm dave not u peter
Peter is my voice when I'm at other functions. His answer is bang on. (The only thing your thread title didn't do was promise I'll give my girlfriend a night she'll never forget.)
 
  • #17
sysreset said:
If the balls are touching they don't move. Mechanical pulses through "rigid" touching objects should travel at the same speed as mechanical pulses through a single "rigid" rod, although perhaps not at the speed of light. Mechanical pulses are not photons. I think mechanical pulse speed depends on the molecular structure of the "rigid" material. I wonder what the fastest mechanical pulse would be through? Perhaps a diamond? How fast would the pulse travel?

(Actually, the billiard balls will move because they are not perfectly rigid. The cue ball will compress one side, the wave of compression will pass through the ball and press on the next ball. In doing so, the centre of mass of the billiard ball will have moved a very small fraction.)

Whether you bother to go into it in this detail or not, the point is that the shock wave will take much longer than 8 minutes - it will take as long as the speed of sound in the material that makes up the billiard balls, which will not be more than a few multiples of the speed of sound in air.
 
  • #18
hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.
 
  • #19
OK, everyone, please stay on topic!

Zz.
 
  • #21
Thoreau said:
hey zarbanx, just ignore ppl like dave. your question was interesting all right (if it really did come out of curiosity). i don't think they comprehend how physics or science for that matter really is done - it's not about remembering formulae but being curious.
Should he ignore the helpful advice on the topic I've been giving since the beginning of the thread, or should he focus on the throw-away comment?
 
  • #22
okkkkkkkkk everyone chill sry dave sry peter let's concentrate on the topic i will change the thread titleP.S--how do we change the thread title
 
  • #23
Well, were there any outstanding issues? It seemed pretty wrapped up.

A rod eight light minutes long is not prefectly rigid. SR forbids it. If one end is moved, the rod will transmit that movement through the length of the rod at no more than the speed of sound in whatever material the rod is constructed from. (The hardest substance known, diamond, will only transmit at 1/25000th of the speed of light.) But even if this rod is constructed of some incredibly advanced, super-hard material, the speed of transmission cannot reach the speed of light. Thus, moving one end of the rod will not cause movement in the other end in any less than eight minutes.
 
  • #24
1 light year long staff, used to point with. (dont do this at home)

Wold a soft push travel in the same speed as a shocking hit? :blushing: edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit

What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?
 
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  • #25
korinteren said:
Wold a soft push travel in the same speed as a shocking hit? :blushing: edit:(Thinking about the case with the billiardballs) perhaps it would just generate mutch longer wavelength but same speed? / edit
Wavelength and frequency are reciprocals. A change in one results in an opposite change in the other.

If you pluck a guitar string softly does it play a lower note than of you pluck it strongly?


korinteren said:
What if a man had a 1 light year long staff made of diomond and pointed to the left and then stright up - using 6 moths (and ignoring the fact that the staff would brake and the man had to be twice as strong as me). Would the other end of the staff travel 2x the speed of light? I guess the answer is no due to some space-time stuff. But how would it look like if observed form a far distance?

The rod would bend. Same reason.

Striking the end of the rod send a compression wave through it. Waving the rod sends a transverse wave through it, just like if it were a stiff jump rope.
 
  • #26
I see

Thanks, DaveC426913. I see that what you say is making perfectly sence. (I wish there was something more exiting to it though, but that's life.)
 
  • #27
russ_watters said:
I would imagine a diamond would be the fastest. Here's the speed of sound for a number of materials:

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/soundv.html

diamond: 12,000 m/s

Thanks for the link Russ! When you look at different "rigid" materials some of them have a speed of sound of of the order of 3000 to 5000 m/s (for copper, aluminum, brass, iron, and glass).

This is kind of mind boggling, because if I understand all the posts properly, you could do a simple experiment by yanking on a 3000 meter long copper wire encased in a lubricated, fictionless sleeve, and the other end would not move for a full second! Same for aluminum, bras, iron, glass, and almost every other imaginable material. I think I am missing something, this does not seem right.

Similarly, using this (flawed?) paradigm, if you launch a 300 meter tall rocket with thrust at one end, the other end would not move for one tenth of a second? I'm not buying this.
 
  • #28
This is kind of mind boggling, because if I understand all the posts properly, you could do a simple experiment by yanking on a 3000 meter long copper wire encased in a lubricated, fictionless sleeve, and the other end would not move for a full second! Same for aluminum, bras, iron, glass, and almost every other imaginable material. I think I am missing something, this does not seem right.

Similarly, using this (flawed?) paradigm, if you launch a 300 meter tall rocket with thrust at one end, the other end would not move for one tenth of a second? I'm not buying this.
You've already experienced this if you've observed the delay between a lightning strike and the thunder it makes. Sound travels very slowly compared with light.
 
  • #29
rotating disk with radius = 1 light year

Perhaps things get more interesting as following thought:
If one had a diomond dish that had a radius of 1 lightyear and stand below its center and slowly start accelerating it :**œ
 
  • #30
korinteren said:
Perhaps things get more interesting as following thought:
If one had a diomond dish that had a radius of 1 lightyear and stand below its center and slowly start accelerating it :**œ

What do you think think would happen ?
 

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