What factors affect the reaction time of a long bar?

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Discussion Overview

The discussion revolves around the hypothetical scenario of a very long bar laid on a frictionless surface, where a person at one end pushes it, and the focus is on understanding the reaction time at the opposite end. Participants explore the propagation of force through the bar, questioning whether it occurs instantly or has a measurable delay, and what factors might influence this reaction time.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the propagation time of force through the bar and whether it is instantaneous or has a delay, suggesting that parameters like density may play a role.
  • Another participant references the speed of light as a limiting factor for the propagation of changes in electromagnetic fields between molecules, implying that the reaction time is not instantaneous.
  • A different participant mentions that the motion of a rigid body is constrained by the speed of sound in the material, noting that no known material has a speed of sound approaching that of light.

Areas of Agreement / Disagreement

Participants express differing views on the nature of force propagation through the bar, with some suggesting it is limited by the speed of sound in the material, while others reference the speed of light in relation to electromagnetic interactions. The discussion remains unresolved regarding the exact factors influencing reaction time.

Contextual Notes

There are limitations in the assumptions made about the hypothetical scenario, including the nature of the material and the conditions under which the bar is considered. The discussion does not resolve the mathematical implications of force propagation or the definitions of terms used.

Magna Visus
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Hello guys,

The title might be slightly misleading, it's not related to general physics. It's a hypothetical question that's far from being realistic of practical;
Suppose you have a very very long bar, and suppose it's layed on a type of material with no friction, a person stands at the 1st extremity A of the bar and pushes it, how will the other extremity B react?
Please note that the length here is theoretical (consider it like 1 AU and the bar is layed on some gigatic planet or whatever), if the action happened at A at t0, how long does it take to reach B?
Does the force that pushed the bar at A have a propagation time through the bar? Or does it happen instantly? And if not, what are the parameters that determines this reaction time? (density of the bar, etc..)
If this question have been answered somewhere else, please put a link so I can follow it's details.

Thanks.
 
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See https://www.physicsforums.com/showthread.php?t=536289
 
Last edited by a moderator:
Thanks for the post.
I suspected it would have to do with the bosons (in this case photon) that travels at c:
" all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light."
 
Magna Visus said:
it's not related to general physics

Nor is it related to quantum physics. Moved.
 
See also https://www.physicsforums.com/showthread.php?t=558506. The motion of a rigid body is limited by the compression wave velocity - or speed of sound in that particular media. Needless to say, there is no known media where the speed of sound is even close to the speed of light.
 

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