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How to calculate the comet's speed?

  1. Dec 9, 2006 #1
    1. The problem statement, all variables and given/known data

    Comet moves around the sun in a very ellipical orbit around the sun. As it approach, in 1986, Comet Halley was 8.79x10^7km from the sun and moving with a speed of 54.6km/s. What will be the comet's speed be when it crosses Neptune's orbit in 2006?

    length of comet from the sun = 8.79x10^7km = 8.79x10^10 m
    v of comet = 54.6km/s = 54.6x10^3m/s
    T = 20 years
    length of neptune from the sun = 4.50x10^12m


    2. Relevant equations

    i use the equation T^2 = (4*pie*r^3)/GM and K2 + U2 = K1 + U1

    3. The attempt at a solution
    i first use the Kelper's rule to find M of the comet, but i don't really know if that's the right step to start with.

    Thank you.
     
  2. jcsd
  3. Dec 9, 2006 #2

    OlderDan

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    Make the substitutions in your conservation of energy equation and see if you really need to know the mass of the comet.
     
  4. Dec 9, 2006 #3
    one question, when we calculate with the K2 + U2 = K1 + U1, the M we are using, is that the mass of the sun or neptune?
     
  5. Dec 9, 2006 #4

    OlderDan

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    The mass of Neptune does not matter. You are just using the radius of the Neptune orbit as a distance from the sun. The potential energy terms depend on the mass of the sun and the mass of the comet.
     
  6. Dec 9, 2006 #5
    i got the answer, thank you so much!

    btw, so does that mean if for similiar questions, the M is always M of sun?
     
    Last edited: Dec 9, 2006
  7. Dec 9, 2006 #6

    OlderDan

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    If the orbit is an orbit around the sun, then the mass of the sun is involved. There are many problems involving orbits around other force centers, so M will not always be the sun's mass.
     
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