# Universal law of gravitation problem

1. Dec 15, 2011

### darlingdarlin

1. The problem statement, all variables and given/known data
Mars has a diameter of .54 times that of Earth and a mass of .11 times that of Earth. Suppose a rover was launched on Earth with the mass of 525 kg. Remember that g is -9.80 m/s/s
A) How much does the rover weigh on Earth?
B) How much does it weigh on Mars?

2. Relevant equations
F= G [(M1)(M2)]/ r^2

3. The attempt at a solution
i have no idea where to start

2. Dec 15, 2011

### Dick

I think you should look up the values for the mass of the earth and radius of the earth and the constant G. Then you should have some numbers to plug in to your equation. Try doing that. Though given g=9.8m/s^2 you could shortcut that. But there's no reason not to do it directly.

3. Dec 15, 2011

### darlingdarlin

i'm trying to understand this problem for my exam tomorrow so i dont think finding those would actually help me...do you know how to do a ratio with it
i did it before but now i forgot how i did it correctly and i cant get the answers that i was supposed to get

4. Dec 15, 2011

### Dick

Ok, the first one should be easy. What does it weigh on earth?

5. Dec 15, 2011

### darlingdarlin

i got 5.15 x 10^3 N

6. Dec 15, 2011

### Dick

Ok. So F_earth= G [(M_earth)(M2)]/ (r_earth)^2=5145N. F_mars= G [(M_mars)(M2)]/ (r_mars)^2. M_mars=0.11*M_earth. r_mars=0.54*r_earth. How does that change the weight? Since M_mars is less than M_earth that decreases it by a factor. What factor? Since r_mars is less than r_earth that increases it by a factor. What factor?

7. Dec 15, 2011