# Homework Help: Gravitational force for a ball and particle

1. Mar 10, 2014

### PKay

1. The problem statement, all variables and given/known data
How far from a very small 110kg ball would a particle have to be placed so that the ball pulled on the particle just as hard as the earth does?

2. Relevant equations

Fg= gMm/r^2

3. The attempt at a solution
I used the equation and plugged in the numbers for the ball and the earth, but then I realized that I needed to use the mass for the particle. However, I don't know what the mass for the particle is.

2. Mar 10, 2014

### PhysicoRaj

It does not require the mass of the particle.

3. Mar 10, 2014

### PKay

Am I supposed to leave it as a variable?

4. Mar 10, 2014

### PhysicoRaj

It's your choice..if you do that, you can see it cancels out ultimately.

5. Mar 10, 2014

### PKay

Thanks! I see it now... I end up getting R= R^2 * Mball/Mearth I'm hoping I'm in the right direction.

6. Mar 10, 2014

### PhysicoRaj

Yes.

7. Mar 10, 2014

### PKay

I get 7.49-10^-10 but it says I'm wrong.

8. Mar 10, 2014

### PKay

Whoops. I'm sorry. I forgot to square root that. Thanks so much for your help! Now to tackle the other 2 monstrous problems.