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Homework Help: Dynamics with Polar coordinates

  1. Jan 24, 2010 #1
    Hey guys,
    I have attached the question with the diagram.
    So far i have found my magnitude of velocity = 90mm/s.
    im just really stuck now, i cant find my angle to find my components Vr and V(theta)
    I also know that you can solve this problem by finding a relationship between theta and "r", with the cosine law, but i have done that method, and i want to figure this out the way that is applicable to all questions. Thnx =)

    Attached Files:

  2. jcsd
  3. Jan 24, 2010 #2


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    Homework Helper

    Hey Larrytsai! :smile:

    (have a theta: θ and a beta: β :wink:)
    General method: just be logical, draw any right-angles you need to help you, and give everything names.

    So draw the perpendicular from A to BC, call the foot of that perpendicular D, and use elementary trig. :smile:
  4. Jan 24, 2010 #3


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    Homework Helper

    According to cosine rule
    r^2 = a^2 + b^2 -2*a*b*cosβ.
    Find the derivative to get dr/dt. Find the second derivative to find d^2r/dt^2
    Next according to sine rule
    r/sinβ = a/sinθ or
    r*sinθ = a*sinβ
    Find the derivative to get dθ/dt. The second derivative will give you the remaining answers.
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