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Homework Help: Calculating the escape speed and gravity of a planet / moon

  1. Jul 20, 2012 #1
    One of the asteroids, Ida, looks like an elongated potato. Surprisingly it has a tiny (compared to Ida) spherical moon! This moon called Dactyl has a mass of 4.20x1016kg, and a radius of 1.57x104 meters, according to Wikipedia.
    - Find the acceleration of gravity on the surface of Dactyl.
    - Find the escape speed on Dactyl.
    - If you are 10,000 meters above the surface of Dactyl, what must your orbital speed be?

    I wanna make sure that I've solved the problem correctly. Can anyone check my work?

    What have I done:
    - Calculate the acceleration of gravity:
    F = (G*M)/R2 = (6.67384*10-11 * 4.20*1016) / (1.57*104)2 = 0.0113717 m/s2 = 11.3717 * 10-3 m/s2

    Escape Speed:
    I saw on wikipedia that the formula for escape speed is:
    ve = sqrt((2*G*M)/r)
    That would give = sqrt((2*6.67384*10-11 * 4.20*1016) / 1.57*104) = 18.8963 m/s

    Orbital Speed:
    Formula for orbital speed:
    vo = sqrt((G*M)/r)
    That would give = sqrt(6.67384*10-11 * 4.20*1016) / 1.57*104 + 10000) = 10.4434 m/s
    Last edited: Jul 20, 2012
  2. jcsd
  3. Jul 20, 2012 #2


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    Staff: Mentor

    Values look okay!

    Be sure to use use an appropriate number of significant figures when you report your results.
  4. Jul 20, 2012 #3


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    [Removed: Question already answered]
  5. Jul 20, 2012 #4
    thx for the replies. The teacher didn't specify how many significant figures...
  6. Jul 20, 2012 #5

    D H

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    Science Advisor

    The teacher doesn't have to say how many significant figures to use. It's right there in the question. How many significant figures are used in the question?
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