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Homework Help: Rotational Motion Question Help

  1. Dec 1, 2016 #1
    1. The problem statement, all variables and given/known data
    So I need to move this rod with length r from point A to B as shown. It has to rotate from A to B which is 90° within 10 seconds. What I want to calculate is the torque and power required if a motor was to produce this motion (location of the motor is shown in the diagram). Can you then also calculate the torque and power required from the motor if this was in space (object is weightless, but the mass is still there). you can ignore friction and other small effects. here's the link for the diagram http://imgur.com/a/egtnK

    2. Relevant equations
    P = T * w
    w - angular velocity
    T - Torque
    P - Power
    3. The attempt at a solution
    So I'm guessing using the above equation we can find power required for the motor?
    So I can calculate angular velocity as follows,
    w = 90/10 = pi/20 radians per second.

    But now how do I calculate torque? What happens in space when there's no weight?

    Thank you in advance
  2. jcsd
  3. Dec 1, 2016 #2


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    The angular velocity cannot be constant. The rod has to be accelerated at some finite rate...

    ... consequently, torque, or power, or both must vary. Is the constraint constant torque or constant power?
    That's the average angular velocity.
  4. Dec 1, 2016 #3

    Simon Bridge

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    Welcome to PF;
    Torque is calculated in the normal way - how would you usually go about it?
    Weight is just the force due to gravitation - when you do not have any weight, just don't include that force in your calculations.

    Note: the problem presented seems to be underspecified:
    You want to get something from position p to position q, and it has an initial and final orientation.
    You also need to know the initial and final speeds as well as the time required for the manouver... and you need to specify if the move is done under constant acceleration or what.

    In general, you divide the motion into translation of the center of mass and rotation about the center of mass... and you need vector equations.
  5. Dec 1, 2016 #4
    so I want the initial velocity and the final velocity to be zero. I want the rod to move from point A to B in 10 seconds.
  6. Dec 1, 2016 #5

    Simon Bridge

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    There are literally an infinite number of ways to do this ... you will need at least two forces and two torques, which each must act over some subset of the 10s time period for the motion. ie. do you want 5s acceleration and 5s deceleration? Or do you want the acceleration and deceleration to be very short - like getting hit by a mallet? (In which case we probably want specific-impulse forms of the laws). What are the constraints? ie. are you limited in available energy for the manouver (ie a rocket would have limited fuel).
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