# Finding the mass of two planets

1. Sep 23, 2006

### prlion16

Ok well, I have used up 2 out of 3 of my submissions for this question. I have tried to do a system of equations and failed twice. I really don't even know where to start so I was hoping that you guys could help me.
The problem is.
The mass of a robot is 5720 kg. This robot weighs 3720 N more on planet A than it does on planet B. Both planets have the same radius of 1.33x10^7 m. What is the difference MA - MB in the masses of these planets?

Thank you.

2. Sep 23, 2006

### neutrino

Use the defintion of weight. Assume that it's evaluated at the surface of the planets.

3. Sep 23, 2006

### prlion16

ok... I still don't get what I am supposed to do. Sorry. I get that weight is going to be calculated at the surface of the planets so d=1.33x10^7. I got the equation F=6.67x10^-11(5720m1/((1.33x10^7)^2)). I really just don't get what I am supposed to do after this.

4. Sep 23, 2006

### neutrino

You are provided with the difference in weight (of the robot), right?

5. Sep 23, 2006

### prlion16

For planet B it is 5720a and Planet A is 5720a+3720. right?

6. Sep 24, 2006

### neutrino

WA - WB = Gm(MA-MB)/d2