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Homework Help: Graviational acceleration given mass and radius

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data
    If you lived on a planet with five times the mass of Earth and twice the radius, what would be the gravitational acceleration at the surface of your planet?

    2. Relevant equations
    Mass of Earth = 6.00*10^24 kg
    Radius of Earth = 6.38*10^3 km

    3. The attempt at a solution
    So, when I started out doing this, I quickly plugged in earth's radius (in m), mass (in kg) and the gravitational constant just to make sure it came out at 9.8 because I know that's what Earth's gravitational acceleration from the surface is, but none of the answer I get are even close. I can't even get this right and I still have to do the actual problem (using 5x the mass and 2x the radius).
  2. jcsd
  3. Oct 31, 2012 #2


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    Science Advisor

    Two rules for doing physics problems:
    1 - Check your units
    2 - Never put in a hard number until you can't avoid it

    You're tripping over rule 1 - which of your numbers is not in SI units?

    Remember rule 2 when you're doing the question proper. The approach I suspect you are about to use will work, but there is a way to do it without needing to know either the Earth's mass or radius.
  4. Oct 31, 2012 #3
    Thanks, I actually went back and redid it later and got the answer I was looking for.
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