# Gravitational Force on different planets

I thought I did this problem correctly but when I submit my answer it says other wise.

Here is the problem: The mass of a robot is 5440kg. This robot weighs 3610N more on planet A than it does on planet B. Both planets have the same radius of $$1.33 * 10^7m$$. What is the difference $$M_{A} - M_{B}$$ in the masses of these planets?

And I my answer must be in kg.

So I know to use:

$$W = G \frac{M_{E}m}{r^2}$$

My answer for the mass on planet B was 12272.1 and for planet A was 13103.1. But when subtracting A - B my answer is wrong.

What am I doing wrong?

Thanks in advance for any help provided.

Last edited:

tony873004
Gold Member
BlackMamba said:
My answer for the mass on planet B was 12272.1 and for planet A was 13103.1.
This doesn't make sense. The mass will be the same on both planets. It's the force or Newtons, or pounds that will change from planet to planet.

Just my guess...
It looks like a simultaneous equation problem.

weight of robot on planet A:

$$W_{a} = G \frac{M_{a} M_{robot}}{r^2}$$

weight of robot on planet B:
$$W_{b} = G \frac{M_{b} M_{robot}}{r^2}$$

Since the robot weighs 3610N less on planet B than on planet A:
$$W_{b} = W_{a}-3610$$

so...

$$G \frac{M_{a} M_{robot}}{r^2} = G \frac{M_{b} M_{robot}}{r^2} - 3610$$

G is known, mass of robot is known, radius is known.
Now solve for Mass of planet A and Mass of planet B.