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Homework Help: Determining the torque and angular momentum of a bullet due to gravity

  1. Apr 15, 2012 #1
    1. The problem statement, all variables and given/known data

    A bullet of mass "m" is fired from a cannon located at the origin. The shell moves in the yz plane, where "z" is the vertical coordinate, with an initial velocity of magnitude "v0" at an angle θ above the y-axis.
    A) What is the torque on the shell, about the origin, as a function of time?
    B) What is the angular momentum of the shell about the origin as a function of time? (Use the definition of angular momentum to solve)

    2. Relevant equations

    Torque = r x F = r*F*sinθ
    Angular momentum = r x p = r x m*v = r*p*sinθ

    3. The attempt at a solution

    My textbook says the answer for A) is -v0mgtcosθ (in the [itex]\vec{i}[/itex] direction) and -(1/2)v0mgt2cosθ ([itex]\vec{i}[/itex])

    I'm primarily having trouble determining the radius from the origin. I want to say it's v0cosθ*t, and I want to say the force acting perpendicular to the radius is "mg", but then my answer becomes -v0mgtcosθsinθ ... Can someone please help?
    Also, I'm not sure how to plug momentum into the angular momentum formula to properly find that for part B.

    Any help would be extremely appreciated.
  2. jcsd
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