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mechanics question on string being light

Discussion in 'Mathematics' started by fudgesweets, Jan 9, 2012.

  1. fudgesweets

    fudgesweets New commenter

    Hi,

    This question has been bothering me:

    Two particles A and B are connect via string and the string passes over a pulley. The pulley is at the top of an inclined plane. The question is
    'The string is described as being light. State how you have used the fact that the string is light in your calculations'
    The answer is 'tension in the string remains constant at both A and B'- i DONT GET THIS...I would have said the mass of the string is not considered in the calculations.....can someone explain to me how the tension answer is reached??
    ...please?
     
  2. fudgesweets

    fudgesweets New commenter

    Hi,

    This question has been bothering me:

    Two particles A and B are connect via string and the string passes over a pulley. The pulley is at the top of an inclined plane. The question is
    'The string is described as being light. State how you have used the fact that the string is light in your calculations'
    The answer is 'tension in the string remains constant at both A and B'- i DONT GET THIS...I would have said the mass of the string is not considered in the calculations.....can someone explain to me how the tension answer is reached??
    ...please?
     
  3. I would have given your answer, and expected to get the mark. However,
    the other answer is also correct. Just think what would happen to a heavy
    string, without weights attached, once it got slightly out of balance.
     
  4. PaulDG

    PaulDG Occasional commenter

    There are two aspects to the assumption.
    Had the string been assumed to have significant mass, then the easy element of the problem to include in the model would have been that mass' effect on the acceleration of the system.
    The more difficult thing to model is the problem of the mass on each side of the pulley changing as the system moves (as pointed out above).
    By just stating "the mass is not considered in the calculations", you're possibly only telling the examiner you were aware of the first problem.
    Stating the Tension is different shows you're aware of the more important one.
     
  5. Paul, I agree with your fuller analysis of the modelling.
    If this is an actual examination question, the principal examiner's comments in the paper report would be interesting.
     

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