Energy Transfer: Instant Transmission?

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Discussion Overview

The discussion revolves around the concept of energy transfer through a solid object, specifically a stick, when it is pushed from one end. Participants explore the implications of speed, material properties, and the nature of kinetic energy transmission, considering both theoretical and practical aspects of the phenomenon.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant claims that pushing a stick from one end results in instantaneous movement of the entire stick, suggesting infinite energy transmission speed.
  • Another participant counters that the impulse travels through the stick at the speed of sound, indicating that energy is not transmitted at or above the speed of light.
  • Doc Al raises a hypothetical scenario about pushing the stick at twice the speed of sound, questioning whether this would lead to observable compression of the stick's length.
  • Further clarification from Doc Al emphasizes that pushing one end of the stick does not result in instantaneous movement, as it takes time for the entire stick to respond.
  • A participant questions whether the absence of friction in space would affect the speed of energy transmission through the stick.
  • Another participant introduces a thought experiment involving marbles in a tube to illustrate kinetic energy transfer, asking about the timing of movement between marbles.
  • One participant acknowledges a realization of their earlier misunderstanding regarding the energy transfer concept.
  • A later reply discusses the complexities of material mechanics, noting that exceeding the speed of sound in a material could lead to plastic deformation and potential disintegration of the stick.

Areas of Agreement / Disagreement

Participants express disagreement regarding the nature of energy transfer through the stick, with some asserting instantaneous movement and others refuting this by referencing the speed of sound. The discussion remains unresolved, with multiple competing views on the implications of speed and material properties.

Contextual Notes

Participants highlight limitations in understanding material mechanics, particularly regarding elastic and plastic deformation, and the conditions under which these phenomena occur. There is also uncertainty about the impact of external conditions, such as friction and environment, on energy transfer.

jamesb-uk
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If you have a stick and push it from one end, the whole stick moves instantly. You have given the other end of the stick kinetic energy. So doesn't this mean that you have transmitted energy at an infinite rate, rather than at or below the speed of light?
 
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jamesb-uk said:
If you have a stick and push it from one end, the whole stick moves instantly.
No it doesn't. The impulse moves through the stick at about the speed of sound. So nothing is being transmitted anywhere near the speed of light, much less infinitely fast.
 
Doc Al:
What if you push it with a speed that's twice the speed of sound- wouldn't that mean that you could see a noticeable difference in the length of the stick, as the atoms were compressed together, if you filmed it?
 
jamesb-uk said:
Doc Al:
What if you push it with a speed that's twice the speed of sound- wouldn't that mean that you could see a noticeable difference in the length of the stick, as the atoms were compressed together, if you filmed it?
I'm talking about how long it takes for a smack given to one end of a stick to travel to the other end. If you push one end of the stick, it takes time for all parts of the stick to get moving. It's not instantaneous. So, yes, the stick will be compressed a bit when you push on one end. (Push too hard and you'll just break it, though. :frown:)
 
What if it's in space and there's no friction- would it be transmitted faster?
 
jamesb-uk said:
What if it's in space and there's no friction- would it be transmitted faster?
Not sure I understand what you're getting at with this question. I see no reason why being in space would have anything to do with it.
 
I'm not talking about vibrations.
Ok, how about if I put it like this:
There is a long tube full of marbles which are all touching each other. I put another marble in at one end and another comes out of the other end. I have transferred kinetic energy through the marbles. How quickly was it transferred, or how long did it take between the original marble at the entry end starting to move, and the exiting marble starting to move.
 
Actually, forget it- I've realized my error.
 
No one answered this part:
jamesb-uk said:
What if you push it with a speed that's twice the speed of sound- wouldn't that mean that you could see a noticeable difference in the length of the stick, as the atoms were compressed together, if you filmed it?
Material mechanics gets a little complicated and I'm not exactly sure how a stick works, but a steel rod has a certain range over which the deformation is perfectly elastic and perfectly reversible. If you try to move it faster than its speed of sound (with as much acceleration as it is willing to absorb), you'll start to move into the range of plastic deformation, whereby the impulse will travel through it faster than its speed of sound but only at the cost of permanent deformation. Much faster than that and the forces trying to accelerate it will exceed the material's strength and it will start to disintegrate.

A stick likely has a much lower and smaller range of elastic deformation and essentially no range of plastic deformation: exceed its speed of sound by more than a little and it'll just shatter.
 

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