Would the other end move instantaenously

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Discussion Overview

The discussion revolves around the propagation of motion in a hypothetical iron bar stretched between the Sun and Earth, specifically whether the other end of the bar would move instantaneously if one end is pulled. The conversation explores concepts related to rigidity, the speed of sound in materials, and the nature of mechanical waves.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that the disturbance in the bar would propagate at the speed of sound, emphasizing that no perfectly rigid objects exist in relativistic contexts.
  • Others argue that the time for the other end to move would be significantly longer than 8 minutes, with estimates ranging from hours to months.
  • One participant questions the calculation of the time it would take for the other end to move, seeking clarification on the equations used.
  • Several participants discuss the nature of sound waves and compression in materials, noting that the movement of one end creates a wave of compression that travels at the speed of sound in the material.
  • There is a debate about why the effect propagates at the speed of sound rather than the speed of light, with some suggesting that atomic interactions and inertia contribute to the delay.
  • Some participants express curiosity about the theoretical underpinnings of why compression waves travel at the speed of sound and seek references for further reading.
  • One participant introduces a hypothetical scenario involving a perfectly homogeneous bar and questions where it would fail under tension.
  • Another participant comments on the implications of the discussion, questioning whether any mechanical linkage can operate faster than the speed of sound.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as multiple competing views remain regarding the nature of rigidity, the speed of wave propagation in materials, and the implications of these concepts in mechanical linkages.

Contextual Notes

Some limitations include the dependence on definitions of rigidity and sound speed, as well as unresolved mathematical steps regarding the propagation of waves in materials.

kevevans
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It Light takes about 8 mins to travel the 93 million miles between the sun and the earth. If there was an iron bar in space along this distance and one end was pulled towards you would the other end move instantaenously ?
 
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The the disturbance would propagate at the speed of sound. You must remember in relativistic problems there are no perfectly rigid objects.
 
No, it will be much much slower than 8 minutes.

Probably a few hours/days/weeks
 
May I ask what equation you use to calculate that?
 
The speed of sound in iron, which is around 5 km/s.
 
Oh my doubt is why it takes months for other end to move? Since iron bar is rigid object it should move instantaneously right?
 
n.karthick said:
Oh my doubt is why it takes months for other end to move? Since iron bar is rigid object it should move instantaneously right?

Wrong.
Nothing is rigid.
All materials consist of atoms which are held together by nothing more than electric fields.

Movement of one end of a metal bar results in a wave of compression - just exactly the same as a sound wave in air (in fact it IS a sound wave), which moves down the length of the bar at the speed of sound for the material.
 
  • #10
n.karthick said:
Oh my doubt is why it takes months for other end to move? Since iron bar is rigid object it should move instantaneously right?
It is more accurate to describe the iron bar as a very stiff spring.
 
  • #11
AJ Bentley said:
Movement of one end of a metal bar results in a wave of compression - just exactly the same as a sound wave in air (in fact it IS a sound wave), which moves down the length of the bar at the speed of sound for the material.
Sorry I don't know anything about this. Could you please explain why it travels at the speed of sound? I could recall that speed of sound is due and depends on the properties of the medium at which it is travelling.
Here, coming to our iron rod example, though it is not rigid and it is like a spring or something like that, what makes it to travel at speed of sound in vacuum (no medium is present)?
Could you specify some references so that I can read and understand pertaining to this.
 
  • #12
Well, it's a tautology really.

Sound is just vibration. The speed of sound is the speed at which vibrations travel.
Vibrations travel at the speed of sound. It's the same statement.

Of course there's a medium - that's the bar.
 
  • #13
n.karthick said:
what makes it to travel at speed of sound in vacuum (no medium is present)?

He didn't say it travels at the speed of sound in vacuum. That wouldn't make any sense because sound doesn't travel at all in vacuum.
 
  • #14
Can anyone explain why the effect propagates at the speed of sound in the bar, and not the speed of light? The interactions between adjacent atoms is electromagnetic, so I would have thought it would be closer to c.

You can imagine giving the bar a very strong tug so that the end near you is moving faster than sound. Would that break the bar at some distance along it? Where? Could you accelerate it gently enough so that it doesn't break?
 
  • #15
Ok, I know sound requires medium to travel but I am curious to know how the compression wave in metal bar travels at speed of sound, that too in vacuum. I mean why it travels at such low speed. Why can't it travel faster than that? What is the theory/ mathematical derivation which shows that it travels at speed of sound. I am asking this, just for the sake of knowing the physics behind it. If you could suggest some material or book it will be helpful to me.
 
  • #16
Tomsk said:
Can anyone explain why the effect propagates at the speed of sound in the bar, and not the speed of light?

Although the actual force travels between atoms at c, it's weakened by their separation and the effects of inertia mean there is a delay between the movements of adjacent atoms. It's exactly analogous with masses connected by (weak) springs.
 
  • #17
n.karthick said:
Ok, I know sound requires medium to travel but I am curious to know how the compression wave in metal bar travels at speed of sound, that too in vacuum. I mean why it travels at such low speed. Why can't it travel faster than that? What is the theory/ mathematical derivation which shows that it travels at speed of sound. I am asking this, just for the sake of knowing the physics behind it. If you could suggest some material or book it will be helpful to me.

Quite simply, because that's the definition of the sound speed.

The speed of sound in any material is the speed of a pressure wave in that material (specifically, the speed of a pressure wave in the limit as the amplitude goes to zero - a large amplitude pulse would travel a bit faster). There's no real derivation, since that's just the definition of the sound speed.
 
  • #18
Something mentioned earlier brought a question to mind.
If the aforementioned iron rod (or rope, or cable, whatever), of any given dimensions, is perfectly homogenous, with no difference in molecular structure or cross-sectional area anywhere along its length, is there a way to determine where it will fail under excessive tension? For instance, if two bulldozers play tug-of-war with it, will it break in the middle, closer to one of them, or is it unknowable in advance?
I have no particular need to know this; it's just curiosity.
 
  • #19
Thanks for all the replies. I never thought that the other end would move instantaenously. However, I am surprised that so many posts suggest that it is linked to the speed of sound. If this is so does it mean that no mechanical linkage can operate faster than the speed of sound. This does not sound (excuse the pun) correct??
 
  • #20
kevevans said:
does it mean that no mechanical linkage can operate faster than the speed of sound.

Right. Keep in mind, though, that for any "normal" scale, sound is very fast.
 
  • #21
russ_watters said:
It is more accurate to describe the iron bar as a very stiff spring.

May I add that as a matter of fact, a spring is itself quite often made out of an iron bar... a small, curled up, iron bar.

Also want to add that if sound waves are slower than light, it can be seen as because each atom has mass, and all these masses take time to accelerate and decelerate. A rough analogy is a set of falling dominos (gravity instead of E field).
 
  • #22
I'd like to break it down a little further...
AJ Bentley said:
Although the actual force travels between atoms at c, it's weakened by their separation and the effects of inertia mean there is a delay between the movements of adjacent atoms. It's exactly analogous with masses connected by (weak) springs.
Not sure what "weakened" means in this sense, but otherwise, yes: On an atom-by-atom basis, it would be modeled as a collection of lots and lots of little spring-mass systems. One atom moves and hits the next, which moves and hits the next, which moves and hits the next...
 
  • #23
Danger said:
Right. Keep in mind, though, that for any "normal" scale, sound is very fast.

Ok what is the speed? 330 metres/sec or even more?
 
  • #24
Tomsk said:
Can anyone explain why the effect propagates at the speed of sound in the bar, and not the speed of light? The interactions between adjacent atoms is electromagnetic, so I would have thought it would be closer to c.

You can imagine giving the bar a very strong tug so that the end near you is moving faster than sound. Would that break the bar at some distance along it? Where? Could you accelerate it gently enough so that it doesn't break?

Imagine a long line of identical toy cars all linked together with springs.If the end car is vibrated to and fro the spring connecting it to the second car is alternately stretched and compressed causing the second car to vibrate to and fro.The motion of the second car is then carried to the third car and so on the result being that the vibrations constitute a longitudinal wave which eventually reaches the car at the other end of the line.With this simple model there are two changes that can make the wave travel faster:
1.Replace the springs with stronger springs so that the vibrations are carried faster between the cars.
2.Replace the cars with cars of a smaller mass so that their accelerations are greater.

Of course this is a simplistic situation but it is a reasonable analogy of how sound travels.In the case of the metal bar the cars are analogous to the metal atoms and the springs are analogous to the interatomic forces.
 

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