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A weight of 15 lbs at sea level is suspended

  1. Aug 31, 2003 #1
    Yes, this is in regards to my first question.

    A weight of 15 lbs at sea level is suspended by a wire rope that does not stretch. The weight is lifted 1 ft and dropped 1 ft. the initial velocity is "zero". What is the peak lb load on the wire when the weight stops falling?

    Thanks for your reply.

    This problem does not require a precise answer. I am looking for a plus or minus 5 lb answer. The missing factors that you have mentioned can be assumed in generalities. Is this enough information to come up with an answer?
     
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Aug 31, 2003 #2

    marcus

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    the peak tension will depend very much on how abruptly the
    weight is assumed to stop falling.

    in a normal problem this would be influenced by the stretch
    of the cable----the less elastic the cable the more abruptly
    the object halts and the greater the force.

    but your problem is not a problem in physics
    since the cable does not stretch, and also it is not well posed since (unless information can
    move faster than light) there is no "when"

    there is no well-defined instant in time "when the weight'
    stops falling"

    To see this, imagine that the cable is attached at the top of
    the weight. the weight must occupy a physical volume, must
    have for example a height, say like 4 inches, between
    the bottom and the top where the cable is attached.

    when the cable snaps taut it stops the top of the weight
    from falling, but the rest of the weight does not know about this
    yet and keeps on falling.

    so the weight itself must stretch slightly (unless information travels thru the weight at infinite speed)

    so the deceleration of the weight is not instantaneous

    if any deceleration is instantaneous one can say that the force applied is infinite---but that is not a physical process

    so the deceleration of the weight is GRADUAL according to some timetable. If you would somehow indicate the timetable for the weight to stop falling and it would be possible to estimate the peak tension "load" as you ask
     
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