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Rotational Motion

  1. Dec 1, 2003 #1
    Imagine a rod in space. If I exert a force at one end, will the rod translate, rotate, or both? How do I determine what it will do?
  2. jcsd
  3. Dec 1, 2003 #2


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    It will rotate about it's centre of mass, and it's center of mass will move in a translation. Essentially, you are applying a moment and an unbalanced force to the body.
  4. Dec 1, 2003 #3
    But what will be the acceleration of its rotation and translation?
  5. Dec 1, 2003 #4


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    Decompose the external force vector into components at the point of application. One componet which passes through the CM of the body the other perpendicular to it. The component through the CM will become a translational acceleration the other component times the distance to the CM will be the torque which cause rotation.
  6. Dec 2, 2003 #5
    So does that imply that when a force is acted upon a lever, like a seesaw, not all of the force acts in the rotation?

    Okay, if I had a seesaw of 2 kg, which was 10 meters long, with a child (4kg) at each end, then what would be the force exerted on the fulcrum. Would it not be 2g + 4g + 4g?

    What would be the force be if the seesaw became to rotate (that is, the net torque is not 0).
  7. Dec 2, 2003 #6


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    Yes, it would be.
    Say for example one kid weighs 2kg (small kid). Then the balanced force acting on the fulcrum is 2+2+2 and the rotational force (moment) is 2.
  8. Dec 2, 2003 #7
    So the translation force is equal to: net force - rotational moment = balanced force? In that case, in the earlier experiment, would the translation force be 0 since the force is not balanced?
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