# Circular Motion

1. Nov 22, 2004

### futb0l

A spacecraft leaves Earth to travel to the moon. How far from the centre of the Earth is the spacecraft when it experiences a net force of zero?

Can anyone help me with this?

Thanks.

2. Nov 22, 2004

### Galileo

Draw an axis. Place the centre of the Earth at x=0 and the centre of the moon at x=R.
Then the gravitational force on a spacecraft of mass m at x between 0 and R is:

$$F=F_{earth}+F_{moon}=-G\frac{M_{earth}m}{x^2}+G\frac{M_{moon}m}{(R-x)^2}$$

3. Nov 22, 2004

### futb0l

Thanks for that :)