# Homework Help: Universal gravity

1. Mar 12, 2009

### tnutty

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

Two identical lead spheres are 14cm apart and attract each other with a force of 0.30(myu)N .

What is their mass?

2. Relevant equations

F = GMm/r^2

3. The attempt at a solution

not sure but :

let myu = u.

.3uN = GMm/r^2

.3umg = GMm/r^2

u = 10/3 *GM/(r^2*g)

not sure what to do next.

Do I use the same equation for sphere 2, but solve for M and substitute?

2. Mar 12, 2009

### phreak

You are confused. N means newtons - it is not a number, it is a unit - it is a standard measurement for force, kind of like centimeters is for length. mg is the force of gravity (on EARTH, which has nothing to do with the problem!), and is not a unit.

This problem is really just plug and chug. You know that the two balls are identical, so that m = M. Otherwise, you're given all the data you need. You don't seem to understand the equation or the concept. Before you manipulate equations, be sure to have an intuitive feel for what the equation actually means.

3. Mar 12, 2009

### tnutty

Fg1 = G*M2 / r2

so,

.30u = G*M2 / r2

and just solve for M?

4. Mar 12, 2009

### tnutty

help, anyone?