# Homework Help: Acceleration due to gravity question

1. Oct 5, 2011

### srikar97

1. The problem statement, all variables and given/known data

the mass of a planet is three times that of the earth and the diameter of the planet is also three times that of the earth.what is the acceleration due to gravity on the surface of the planet.value of g on earth's surface =9.80m/s^2

2. Relevant equations

g=GM/R^2

3. The attempt at a solution
g=GM/R^2

g'=GM'/R'^2

g'/g=(M'/M)(R^2/R'^2)

M'/M=3

R/R'=1/3, g=9.80 m/s^2

g/g'=3*1/9

but the newton's laws of gravity say that mass is directly proportional to gravitational acceleration ,the mass of the planet is 3 times that of the earth so g'>g but her
g'<g i don't know where i have gone wrong perhaps i have done some wrong calculations i am quite doubtful about the answer any help would be greatly appreciated

2. Oct 5, 2011

### Harrisonized

I don't see anything wrong. What's the "correct" answer?

3. Oct 5, 2011

### srikar97

so am i correct? but i don't think it obeys newton's laws of gravitation.. as it says more mass the more the gravitational acceleration on that planet....