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Rod hanging of a string

  1. Nov 13, 2004 #1
    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!
  2. jcsd
  3. Nov 13, 2004 #2
    "Be in the same direction" is a little unclear to be honest. Think about seperate 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 thier 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
  4. Nov 13, 2004 #3
    also... you said "to be in the same direction all the time"... what is changing over time to make any difference?
  5. Nov 15, 2004 #4
    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. !!!!!!!
  6. Nov 16, 2004 #5


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    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...
  7. Nov 16, 2004 #6


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    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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