How would a long object move?

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Discussion Overview

The discussion centers around the hypothetical movement of a long, inflexible rod, specifically one measuring 11.8 million miles in length. Participants explore the implications of applying force to such an object and the nature of motion transmission through it, considering concepts such as wave propagation and the limits imposed by the speed of light.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how a long object would move if force is applied at one end, considering the speed of light as a limiting factor and whether the motion would manifest as a wave.
  • Another participant asserts that an inflexible rod cannot exist and suggests that a real rod would transmit motion through longitudinal waves, specifically sound waves.
  • A different viewpoint posits that if the rod were made from a neutron star, it would still take a significant amount of time for the opposite end to move, speculating that both ends would move at the same speed regardless of external gravitational fields.
  • There is a discussion about the molecular composition of materials affecting the speed of motion, with references to "mechanical information" traveling in sound waves and questioning if there is a theoretical limit on the speed of sound.
  • Some participants note that the speed of sound is limited by the speed of light in a vacuum due to electromagnetic interactions between atoms.

Areas of Agreement / Disagreement

Participants express disagreement regarding the existence of an inflexible rod and the nature of its movement. Multiple competing views remain on how motion would propagate through such an object and the implications of material properties on this motion.

Contextual Notes

Limitations include assumptions about the existence of an inflexible rod and the nature of wave propagation in different materials. The discussion does not resolve the theoretical implications of these assumptions.

Edward Solomo
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How would a "long" object move?

My question is seemingly simple.

Let us suppose that we had an unbreakable/inflexible rod whose length was 11.8 million miles in length (one light-minute). Now assume that we applied enough force at one end of the rod to move the entire rod altogether. As nothing can travel faster than the speed of light, how long would it take for the opposite end of the rod to move? Does the motion of the rod become a wave?
 
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An inflexible rod wouldn't move because it doesn't exist. But yes, a real rod begins to move by pasing the force through the rod in the form of a longitudinal wave: a sound wave.
 


Sure. Let's say this rode is made from a chunk of a neutron star. Well, if this is the case, it will still take more than a second (several hours i guess) before the other end moves. But for a hypothetical stiff, rigid rode I think it can never be at rest neither can it be accelerated. This means at any given time both ends are moving at the same speed irrespective of any gravitavion fields. Think ether. NOTE: This is my own speculation. It should not be taken as fact. Thanks.
 


russ_watters said:
An inflexible rod wouldn't move because it doesn't exist. But yes, a real rod begins to move by passing the force through the rod in the form of a longitudinal wave: a sound wave.

This suggests that the molecular composition would affect the "speed of motion" of a rod.

After doing a little reading (now since I know what to read), I saw some sources calling it "mechanical information" that travels in sound waves, as the guy said above. Is there any theoretical limit on the speed of sound?
 


Edward Solomo said:
This suggests that the molecular composition would affect the "speed of motion" of a rod.
Yes, a rubber rod beheves differently than a steel rod, when rotated.

Edward Solomo said:
After doing a little reading (now since I know what to read), I saw some sources calling it "mechanical information" that travels in sound waves, as the guy said above. Is there any theoretical limit on the speed of sound?
Yes, the speed of light in vacuum. The electromagnetic interactions between the atoms of the rod, cannot go faster than that.
 

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