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Homework Help: Speed of body traveling in elliptical path

  1. Sep 30, 2007 #1
    1. The problem statement, all variables and given/known data
    A body follows an elliptical path defined by r = sd/(1-s*cosP), where s and d are constant. If the angular speed is constant (dP/dT = w), show that the body's speed is v = rw[1+{(r*sinP)/d}^2]^(1/2)

    2. Relevant equations
    v = dr/dT*rhat + r*dP/dT*Phat; P = theta, I am not sure how to insert a theta symbol

    3. The attempt at a solution
    Right now I am thinking that I should find dr/dt. But as I see it since, there is no time dependence in the original equation, dr/dt = 0. Am I safe to take the derivative of r and use it as v? I am thinking no, since v is the magnitude of the velocity vector. Thanks for the help!
  2. jcsd
  3. Sep 30, 2007 #2


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    You've got the parts of it right, but you're a little confused.

    [tex] \vec{v} = \frac{d}{dt} (r \hat{r} ) = r \omega \hat{\theta} + \hat{r} \frac{dr}{dt} [/tex]

    From this, you can write down the magnitude of the velocity, [itex] | \vec{v} | [/itex]

    The only missing piece, is to evaluate dr/dt. This you can do from the elliptic equation, with the time dependence embedded in [itex] \theta(t) [/itex].
  4. Sep 30, 2007 #3


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    Use the chain rule. d/dT=d/dP*dP/dT. P is a function of time and so is r through it's dependence on P.
  5. Sep 30, 2007 #4
    I figured it out, I was stuck after I took the derivative, then i solved for r/d and the answer was right there in front of me! Thanks for the help!!
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