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Homework Help: A few Conceptual Questions Regarding Torque & Angular Momentum

  1. Jul 6, 2012 #1
    1. The problem statement, all variables and given/known data

    The following statements are true/false

    1) The net torque on an object is equal to its moment of inertia times angular acceleration.
    2) Angular momentum will be conserved if there are no external torques acting on it.
    3) When its linear momentum is zero, the angular momentum of a solid object is also zero.
    4) Any force acting on an object can change the objects angular momentum.
    5) Torque is equal to the perpendicular force applied through any lever arm.

    2. Relevant equations

    Sum of torques = I*alpha
    L = I*omega
    L = r*p*sin(theta)
    T = F*r*sin(theta)

    3. The attempt at a solution

    1) True: Positive on this one, seen in equation T = I*alpha
    2) True: Angular momentum is a conserved quantity and cannot be changed from within system
    3) True?: Not sure on this one, I figured since they are analogous properties and L = r*p*sin(theta) it would be true.
    4) False: Only external forces and torques can change angular momentum
    5) False: It is not always the perpendicular force at the lever arm as seen by the equation T = F*r*sin(theta), where theta is not necessarily 90 degrees.

  2. jcsd
  3. Jul 6, 2012 #2
    For #3 think about two bodies orbiting each other due to their mutual gravitation. Angular momentum is a conserved quantity and therefore constant. Hint: The system can also translate via its center of mass.
  4. Jul 6, 2012 #3
    Ah, so it would be false, because even if its linear momentum is zero (object not moving), the object can still rotate around its center of mass, creating rotational momentum.
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