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Torque acting on a projectile

  1. Sep 16, 2015 #1
    In class our teacher shows a problem where he finds out the torque acting on a projectile at the highest point of its trajectory.
    He calculates the horizontal displacement from the vertical axis to the highest point as "r "and multiplies with "mg" as "F".
    Here I am a little confused. Isn't torque associated with rotational motion like "r" is the perpendicular distance from the rotational axis ? How we can treat a projectile motion as a rotatinal motion ?
     
  2. jcsd
  3. Sep 16, 2015 #2

    mfb

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    Torque from what, relative to what?

    From gravity, relative to the center of your coordinate system: well, that is true, but I don't see the relevance of that value as the projectile does not perform a typical rotation around this point.
     
  4. Sep 17, 2015 #3
    Thanks for replying...yes...he mentioned it, moment of the force 'mg'.
     
  5. Sep 17, 2015 #4

    A.T.

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    Not in general. Torque is the time derivate of angular momentum, which doesn't require rotational motion.
     
  6. Sep 17, 2015 #5
    Yes, torque = dL/dt ; L = angular momentum
    But again angular momentum is connected to rotation of object . We do not find angular momentum of linear motion.
    We define an axis of rotation and take the distance from it to measure L = r × mv
    We also know L = I.ω ; I = moment of inertia , ω = angular velocity
    So angular momentum is about rotational motion right ? Or where have I got it wrong ?
     
  7. Sep 17, 2015 #6

    mfb

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    Sure we do, for axes that do not cross the line of motion.
    This is a pointless approach in most situations, but it is not wrong.
     
  8. Sep 17, 2015 #7

    A.T.

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