# Homework Help: Universal gravitation: mass of 2 objects

1. Jun 21, 2010

### mickellowery

1. The problem statement, all variables and given/known data
Two objects attract each other with a gravitational magnitude 1.00x10-8N when separated by 20.0cm. If the total mass of the two objects is 5.00kg what is the mass of each?

2. Relevant equations
Fg=G*m1m2/r2
G=6.673x10-11Nm2/kg2
I'm not real sure how to go about this one since they don't say whether or not the two masses are equal. The answers turn out to not be equal which killed my idea of how to set this up. I was going to try making one of the masses 5/2 and solving for the other mass but this just gives me half of the total mass.

3. The attempt at a solution

2. Jun 21, 2010

### Staff: Mentor

If the mass of one of the objects was 'm', what would be the mass of the other?

3. Jun 21, 2010

### mickellowery

Thats a great question Doc. Would it just be m/2? I'm not very sure.

4. Jun 21, 2010

### Staff: Mentor

No guessing. Figure it out. Hint: What's the total mass?

Here's how to look at it. You've already called the masses m1 and m2. That's perfectly fine. You already have one equation: Newton's law of gravity. But since you have two unknowns, you'll need a second equation. Hint: m1 + m2 = ??

5. Jun 21, 2010

### mickellowery

Oh OK so then it would be m1=5-m2. Then I would sub in 5-m2 for m1

6. Jun 21, 2010

### Staff: Mentor

Yep. Then you'd solve for m2 (and then for m1).

7. Jun 21, 2010

### mickellowery

Alrighty thanks much for helping a nearly lost cause in physics.