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Homework Help: Comets Velocity

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data

    The orbit of Halley's Comet around the Sun is a long thin ellipse. At its aphelion (point farthest from the Sun), the comet is 5.7 * 10^12 m from the Sun and moves with a speed of 11.0 km/s. What is the comet's speed at its perihelion (closest approach to the Sun) where its distance from the Sun is 8.4 * 10^10 m?

    2. Relevant equations


    3. The attempt at a solution

    do the m and g values cancel out when doing this because they are not given? Im totally lost...
  2. jcsd
  3. Oct 21, 2007 #2


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    Yes, KE = 1/2 m v^2 and PE = M m /r where M is the mass of the sun.
  4. Oct 21, 2007 #3
    Kepler's 2nd Law is equivalent to the conservation of momentum.

    r[a]mv[a]sin(x) = r[p]mv[p]sin(x)

    As you can see, the mass of the central body is irrelevant and the masses of the comet cancel out.
  5. Oct 21, 2007 #4
    what is the meaning of sin(x) in both of these? it gives no angles
  6. Oct 22, 2007 #5
    The momentum (mv) is perpendicular to the comet's path. Therefore, the angle between the r position vector and the momentum vector is 90. The sine of 90 is 1.
    Last edited: Oct 22, 2007
  7. Oct 22, 2007 #6
    ok I tried that and got 746.4285 and that is still not the right answer...I dont know what I am doing wrong....
  8. Oct 22, 2007 #7
    Do you get the same answer when you try conservation of energy?
  9. Oct 22, 2007 #8
    Oh, you made another topic. mdk is right, [itex]r_a m v_a = r_p m v_p[/itex] so you must have misinterpreted something he said or done the numbers wrong.
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