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Homework Help: Help!(Pendulum in a falling elevator)

  1. Mar 29, 2005 #1
    One of our homework questions is:
    You are in an elevator at the top of the CN tower. You have a pendulum and allow it to oscillate. The elevator falls to the ground after someone cuts its support cables. What does the pendulum do? Ignore the air resistance acting on the pendulum and the elevator

    My teacher suggested making our own pendulum and dropping it (which I did). I found that the bottom of the pendulum went upwards. However, isn't this simply due to air resitance, as the mass of the bottom is lighter than the thing it's attached to? Our teacher recommended trying it at different velocities, but I'm really not sure how to go about this without air resistance being a factor. My temptation is to say that the pendulum would stay in the position it was at when the cables were cut.
    Do I even sound close??? I'm so lost! :surprised

    Any tips or help would be greatly appreciated... :smile:
    Last edited: Mar 29, 2005
  2. jcsd
  3. Mar 29, 2005 #2


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    I think the way to think about it is... how would the pendulum behave if there was no gravity. That's how the pendulum will behave as seen by someone falling inside the elevator with the pendulum. It'll be like you're in outer space with the pendulum.

    You'll have to take into account the velocity of the pendulum just before the cable was cut.
  4. Mar 29, 2005 #3


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    You're close. When free falling, you don't feel the force of gravity. If there is no force on the pendulum, what will it do? (hint: this depends on where it is in its cycle when the cable is cut)
  5. Mar 29, 2005 #4
    The whole system is falling at the same speed after the elevator starts to drop. This means that relative to the system there are no forces other then tension. This means that the pendulem will retain the angular velocity that it had when the elevator started falling.
  6. Mar 30, 2005 #5


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    Oy! What tension? The "support" for the pendulum is also falling at the same rate. The whole elevator is considered as "weightless".

    StatusX and learningphysics responses are correct.

  7. Mar 30, 2005 #6

    I think he's assuming the pendulum is suspended from a massive object. if so, you'd need tension even without gravity.

    ps: unless the pendulum were at it highest point (zero angular momentum) at the instant the elevator started to fall. then there's be no tension.
    Last edited: Mar 30, 2005
  8. Mar 30, 2005 #7


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    Yes, jdavel and Davorak are right. There is tension, unless you're at the highest point.

    Although the whole elevator is considered weightless, the pendulum will be in motion as seen from inside the elevator, unless you're at the highest point when kinetic energy is all converted to gravitational potential energy.
  9. Mar 30, 2005 #8
    Davorak is right. And therefore, pinkie, what do you think the pendulum bob will do? There are two answers; one, if it was at the top of its swing at the instant the cable was cut, and the other, if it was swinging when the cable was cut.
  10. Mar 30, 2005 #9
    Okay, I'm quite confused, but here's what I think happens:
    At zero velocity (ie, the highest point), to the observer in the elevator, the bob will remain where it is.
    At any other velocity, the bob will continue travelling until it hits zero velocity and then stay there....
    My teacher said that this question mostly deals with inertia, which gives me the impression that if the pendulum is at a velocity not equal to 0, that it will keep moving...
    Am I even remotely close...???

    Ps - I really really appreciate the help on this one... :smile:
    Last edited: Mar 30, 2005
  11. Mar 30, 2005 #10
    You're right about the first one.

    As for the other, why will it ever hit zero velocity? (other than when the elevator hits bottom :devil: )

    Look at it another way. When the elevator starts falling it's just as if gravity has been switched off. So instead, imagine that you're in deep space where any gravity is negligible. What happens if you're holding a pendulum & you give the bob a push?
  12. Mar 30, 2005 #11
    I would think that it would follow the direction you pushed it in...if there is no gravity and you pushed it, would it not spin all the way around?
  13. Mar 30, 2005 #12
    Ya got it, baby!
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