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Homework Help: Question about free falling

  1. Dec 26, 2016 #1
    1. The problem statement, all variables and given/known data
    A pendulum is swinging inside a lift. What happens to the bob if the lift cable breaks when the bob is at the lowest point?

    2. Relevant equations

    3. The attempt at a solution
    When I consider the bob relative to the lift, the bob is apparently weightless. There should be no net force acting on the bob as observed by the person in the lift. However, the answer state that there will be a uniform circular motion relative to the falling lift. Where does the centripetal force come from??
  2. jcsd
  3. Dec 26, 2016 #2


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    Staff: Mentor

    Ask yourself what the motion of the bob was at the instant the lift's cable broke. If the pendulum string was not there, what would be the motion of the bob?
  4. Dec 26, 2016 #3


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    Science Advisor
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    Gold Member

    Where did it come from before the lift cable broke?
  5. Dec 26, 2016 #4
    Very skill full answers from gn and CW designed to get clara to think ...now a day has passed I'll spell it out , and make sure I understand it myself....

    At the moment the cable broke 'gravity' experienced becomes zero .... the bob has a horizontal velocity vector , but the string is still pulling the bob...

    In the absence of 'gravity' the bob will swing around the anchor point , forever in a circle, with the same speed.

    If the cable had snapped when the bob was at it's highest point in the swing , no bob velocity, no tension in the string the bob would just float motionless inside the falling lift...

    But what if the cable snapped when the bob was halfway between these two extremes . Any ideas Clara? ....Anyone?
    Last edited: Dec 26, 2016
  6. Dec 26, 2016 #5
    I think it will still undergo circular motion, just like the ball did at its lowest point, but with a lower speed.

    In the previous example, when the moment where the bob is snapped at its lowest point, I don't think there is tension pulling it. The bob moves forward because of inertia which causes the string to pull it back into circular motion afterwards. Am I right or not?
  7. Dec 26, 2016 #6
    I think that's right ... at all other places where the the bob is , when the lift cable breaks , it will have some velocity , so will rotate about the anchor point, but more slowly .....

    For any pendulum , or bob , the tension in the string must be maximum when the bob is lowest , and moving fastest .... The string feels the full weight of the bob and the centrifugal force at a maximum.... at the the instant the lift cable breaks there is no weight of the bob , but still the centrifugal force pulling on the string ..

    Yes that's right ...the bob tries to move forward horizontally but is pulled into a circular movement by the string .
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