# Homework Help: Gravitational problem

1. Mar 11, 2009

### nns91

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

The mass of the earth is 5.98*10^24 kg and its radius is 6370 km. The radius of the moon is 1738 km. The acceleration of gravity at the surface of the moon is 1.62 m/s^2. What is the ratio of the average density of the moon to that of the earth ?

2. Relevant equations

New's law of gravity: F= G*m1*m2/r^2

3. The attempt at a solution

I understand that I need to find mass of the moon to get its density. How should I calculate the mass of the moon ??

2. Mar 11, 2009

### Dr.D

Assume a 1 kg test mass and write a statement about what is true at the surface of the earth and a similar statement for the same 1 kg mass at the surface of the moon.

3. Mar 11, 2009

### nns91

On the earth it will have the weight of 9.81 N and on the moon its weight gonna be 1.62 N. Is that what you are talking about ?

4. Mar 11, 2009

### nns91

Can I use Newton's Law of Gravity here ? But I don't know either F or r and one of the m though