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Homework Help: Ly need help on gravitation only one part

  1. Jan 15, 2010 #1
    urgently need help on gravitation only one part

    Comets travel around the sun in elliptical orbits with large eccentricities. If a comet has speed 7.0 x 10^4 m/s when at a distance of 2.5 x 10^11 m from the center of the sun, what is its speed when at a distance of 9.0 * 10^10 m?
    ___________ m/s

    Attempt
    (7.0 x 10^4)(2.5 x 10^11)= v(9.0 * 10^10)
    v = 194444.444

    However, this answer was marked wrong.
     
  2. jcsd
  3. Jan 16, 2010 #2

    ehild

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    Homework Helper

    Re: urgently need help on gravitation only one part

    First: in what units is your v given?

    The angular momentum is constant along the orbit, but its magnitude is r*v* sin(angle between the radius and the direction of the velocity). This angle is unknown. So use the conservation of momentum instead.

    ehild
     
  4. Jan 16, 2010 #3
    Re: urgently need help on gravitation only one part

    v is in m/s. And what do you mean by saying that i should use coservation of momentum. Isn't that what i did but it was marked wrong?
     
  5. Jan 16, 2010 #4

    ehild

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    Re: urgently need help on gravitation only one part

    Sorry, I meant conservation of energy...

    ehild
     
  6. Jan 16, 2010 #5
    Re: urgently need help on gravitation only one part

    okay but if i use conservation of energy i still encounter a problem.

    KE + PE = KE +PE
    1/2mv^2 + (-GMm/r) = 1/2mV^2 + (-GMm/R)
    1/2v^2 + (-GM/r) = 1/2V^2 + (-GM/R)

    I know
    v = 2 x 10^4 m/s
    r= 3 x 10^11 m
    R=4 x 10^10 m

    but i still need to know what M, the mass of the comet is equal to.
     
  7. Jan 16, 2010 #6
    Re: urgently need help on gravitation only one part

    Isn't M the mass of the sun?
     
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