Calculating Gravitational Acceleration at a Planet's Surface

[dinhjeffrey]

Homework Statement

http://img26.imageshack.us/i/1113001.jpg/

#13

Homework Equations

m=F/a
F=g(m1m2)/r^2

The Attempt at a Solution

well, the answer is D, but I am not sure how a planet's size can affect an object's weight on there. I am guessing you have to use the gravitational constant. I am guessing you plug in the planets weight and the objects weight into m1 and m2 of gravitational constant?

 

[Andrew Mason]

Homework Statement

http://img26.imageshack.us/i/1113001.jpg/

#13

Homework Equations

m=F/a
F=g(m1m2)/r^2

The Attempt at a Solution

well, the answer is D, but I am not sure how a planet's size can affect an object's weight on there. I am guessing you have to use the gravitational constant. I am guessing you plug in the planets weight and the objects weight into m1 and m2 of gravitational constant?

It is all about that r^2 term. If you know M (mass of planet) but don't know r (the radius of the planet - distance from centre to the surface), how do you find a (= GM/r^2) at the surface?

AM