Unsolvable: The Physics of a Hanging Rod and String Dilemma Explained

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Discussion Overview

The discussion revolves around a physics problem involving a rod hanging from a string and the conditions under which both can align in the same direction. Participants explore the implications of forces acting on the system, the nature of the setup, and the degrees of freedom involved in the motion of the rod and string.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the problem and questions why the rod and string cannot be aligned in the same direction, seeking a proof for this assertion.
  • Another participant suggests that the phrase "be in the same direction" is unclear and proposes considering the separate components of the rod and string, indicating that under ideal conditions, they could hang straight down along the y-axis.
  • A different participant questions what changes over time that would affect the alignment of the rod and string.
  • One participant emphasizes the need for clarity in the question, suggesting that an unclear question cannot yield a clear answer.
  • Another participant reiterates the original question and introduces the concept of the system behaving as a multiple pendulum, highlighting the complexity of the degrees of freedom involved.
  • One participant interprets "be in the same direction" as the string and rod acting as a single longer pendulum and challenges others to prove why this alignment is not possible.

Areas of Agreement / Disagreement

Participants express differing interpretations of the problem and the conditions affecting the alignment of the rod and string. There is no consensus on the clarity of the question or the implications of the physical setup.

Contextual Notes

Participants note assumptions such as the ideal conditions of the setup, including uniform density and the absence of external forces, which may not hold in practical scenarios. The discussion also highlights the complexity introduced by multiple degrees of freedom in the system.

Feynmanfan
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I don't know how to solve this rigid solid problem

Let there be a rod hanging of a string fixed to a certain point. Why is not possible for both the string and the rod to be in the same direction all the time. (it is common sense that it's impossible but how can I prove it?)

Thanks a lot!
 
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"Be in the same direction" is a little unclear to be honest. Think about separate x,y and z components of the rod and the string. Assuming the setup is perfect (i.e the string is attached to the rod at the center of one of its ends, no exturnal forces are acting on it other then gravity, uniform density of both parts etc) there is no reason why it wouldn't hang straight down their y axis. If there was to be any exturnal force... i.e wind, then the rod (being of bigger size) would exprience a larger force and hence you would get turning forces which would change the rods direction. Maybe I'm missing something here
 
also... you said "to be in the same direction all the time"... what is changing over time to make any difference?
 
I think that the others said it you cannot get a clear answer to an unclear question --- plus your name sake would NEVER do that. !
 
Feynmanfan said:
I don't know how to solve this rigid solid problem

Let there be a rod hanging of a string fixed to a certain point. Why is not possible for both the string and the rod to be in the same direction all the time. (it is common sense that it's impossible but how can I prove it?)

Thanks a lot!
Basically, in this case, you can regard the string as a MULTIPLE pendulum (more correctly a chain), each component pendulum having one degree of freedom. Adding to that the degree of freedom the stiff rod has, what you're basically asking is why a system with multiple (practically, infinite) degrees of freedom won't behave as a system with only one degree of freedom...
 
I believe by "Be in the same direction" he means the string and pendulum stay lined up, acting as a single longer pendulum. Prove why they won't.
 

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