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Swinging mass on a string + Oscillations

  1. Oct 28, 2011 #1
    If you have a mass on a string and you spin it in circular motion parallel to the plane of the horizontal floor, is the mass falling under the effect of gravity at all? Is it that during this circular motion, the mass falls a certain height and the tension in the string pulls it back up? Then wouldn't the mass be "oscillating" between falling and being pulled back up at that horizontal level?
  2. jcsd
  3. Oct 28, 2011 #2
    yes, gravity is trying to bring the mass towards the vertical, but the centripetal force is counteracting it.
  4. Oct 28, 2011 #3
    You can make it oscillate definitely, but it does not necessarily oscillate. If you control the tension well, where it provides suitable centripetal force while creating enough upward component, then it would not oscillate. Because there is no vertical net force thereby remaining in equilibrium in vertical direction.
  5. Oct 29, 2011 #4
    You will find that after enough time your object will likely 'settle' on an angle from the horizontal inversely proportional to the angular velocity, you can see this by spinning your object slowly and observing that the object does not rise very much, but when spun quickly it will approach parallel to the floor, though it will never (no matter how fast you spin the string) actually be parallel. At first it may oscillate but the damping of the air will stop this eventually.
  6. Oct 29, 2011 #5
    I think I know what you mean by controlling the tension well, but that would mean you're varying the tension to cause the circular motion to be horizontal. What if the tension was perfectly constant and not variable so that only gravity acts downward on the object when it is spinning horizontal to the floor at a given instant?

    Can you explain why it will never actually be parallel to the floor and also the damping situation?
  7. Oct 30, 2011 #6
    It was wrong of me to say it wont reach parallel, what I should have said was that it wouldn't 'sit' parallel, by that I mean when the string is parallel the tension will have no vertical component so the only force acting vertically is gravity, this would make your mass drop slightly, changing the angle of the string so that it does indeed have a vertical component which would pull it back up and so on.
  8. Oct 30, 2011 #7

    Doc Al

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    Staff: Mentor

    What do you mean by 'falling under the effect of gravity'? If the angle of the string is constant--the mass is moving in a horizontal circle--then the vertical component of the tension exactly counters gravity. There is no vertical acceleration. The net force on the mass will be centripetal.
  9. Oct 30, 2011 #8
    I am loosing you guys....

    I guess it is not 100% clear what 'it' means in the OP's initial statement:
    spin what? the mass or the string? I understood the mass, because the sentence starts with "if you have a mass".

    I never understood that the pivot was being spun around to make the mass rotate.

    The way I understood the system was with a fixed pivot where the string is hanging from and the mass had been given an initial velocity to start rotating and that's it.

    In the latter case, because of air drag and friction...the mass will have less and less energy and spin with less and less velocity, reducing its centripetal force and spinning with the string's angle closer and closer to the vertical.

    Now, if you are spinning the pivot of the string, that's another matter where you are actually injecting energy into the system and you could possibly keep the mass rotating and never fall...

    Just wanted to bring the two scenarios to light since I getting mixed signals from the previous posts.
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