Explaining Rigid Body Movement with 15*3*10^8m Stick

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Discussion Overview

The discussion revolves around the movement of a hypothetical rigid body, specifically a stick of length 15*3*10^8 meters, and the implications of pushing one end of the stick on the motion of a ball placed at the other end. The participants explore concepts related to the speed of force transmission, the nature of rigid body movement, and the limitations imposed by the speed of light and sound.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how information can be changed instantaneously when pushing the stick, given that light takes time to travel to the ball.
  • Another participant asserts that pushing one end of a rigid body does not result in instantaneous movement of the other end, indicating that it takes time for the force to propagate.
  • A further inquiry is made about whether the ball would move 15 seconds after the push, suggesting that the force travels at the speed of light.
  • In response, a participant clarifies that the force actually travels at the speed of sound within the material of the stick, and discusses the implications of exceeding this speed.

Areas of Agreement / Disagreement

Participants generally agree that the movement of the ball is not instantaneous and that there are limits to how quickly forces can propagate through materials. However, there is disagreement regarding the specifics of how these limits apply in this scenario, particularly concerning the speeds involved.

Contextual Notes

Participants reference concepts from Newtonian physics and Einstein's theory of relativity, indicating a complexity in understanding the relationship between force transmission and the speed of light. There are unresolved assumptions about the nature of rigidity and the material properties of the stick.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in classical mechanics, the nature of rigid bodies, and the implications of relativistic physics on everyday phenomena.

arunrajagopal
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Assume a stick
of length 15*3*10^8 metre.
One end points towards
me, at the other end we
place a ball very near to
the end of the stick. If i
push the end pointing
towards me with some
force, instantaneously the
ball will be moved(~). At
the same time, a light
beam emanating
alongside me would take
15 seconds to reach that
ball. How can i change the
information of a space
even before the light beam
reaches out to that place
(~)? This question appears
to be childish, pardon my
innocence, explain me
please. I've been baffled
by this question since my
childhood. I know this
question is a flaw or
immature way of
understanding a concept,
please find out the
misconception I've been
thinking of.

P.S: I'm NOT gonna
SHAKE, or BANG, or MAKE
VIBRATE the stick. This situation is slightly analogous to Newton's gravity model which contradicts with light's speed limit later overturned by einstein's general theory of relativity, but not completely i think.

(~)-Doubtful.
 
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arunrajagopal said:
If i
push the end pointing
towards me with some
force, instantaneously the
ball will be moved(~).
When you push one end of the 'rigid' body, it takes time for the other end to move. It's not instantaneous.
 
Doc Al said:
When you push one end of the 'rigid' body, it takes time for the other end to move. It's not instantaneous.

That means the ball will move 15 seconds later after you pushed the rod? The force itself is also traveling at the speed of light?
 
No, at the speed of sound. DaleSpam posted a nice FAQ reply to this question yesterday.

https://www.physicsforums.com/showthread.php?t=536289

If you hit the surface closest to you so hard that it's forced to move faster than the speed of sound in the material, it will probably break. It will also create a disturbance that propagates faster than the speed of sound for a little while (but obviously never faster than the object that hit it), and eventually turns into a plain old sound wave when it has lost enough energy.
 
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