# Graviational acceleration given mass and radius

1. Oct 31, 2012

### gh_pluvilias

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
GM/r^2
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. Oct 31, 2012

### Ibix

Two rules for doing physics problems:
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.

3. Oct 31, 2012

### gh_pluvilias

Thanks, I actually went back and redid it later and got the answer I was looking for.