# Lagrange point

1. Oct 31, 2005

### sony

So im supposed to find the distance from the moon where the force on a particle from the earth and sun is equal.

I get this expression (earth-moon distance is 400 000km):
yMm/(r^2) = yMm / (400000km-r)^2

But from that I get r=200 000km and the answer should be 40 000km...

whats wrong with the expression?

thanks

2. Oct 31, 2005

### µ³

mass of the moon != mass of the earth

3. Oct 31, 2005

### sony

but the product of the masses is the same on both sides right? dont they cancel...?

4. Oct 31, 2005

### Tide

No, they are not the same and they can't cancel if they are not the same.

5. Oct 31, 2005

### sony

then what is the expression...?

6. Oct 31, 2005

### DaveC426913

There should be only one mass variable on each side of your equation.

7. Oct 31, 2005

### sony

but newtons gravitation law is yMm/r^2...

8. Oct 31, 2005

### Tide

Yes. But do you KNOW what M and m are?

9. Oct 31, 2005

### sony

M=mass of earth
m=mass of moon

?

10. Oct 31, 2005

### sony

oh, heh, I don't know what caused me to be so slow... I forgot the mass of the particle at the Lagrance point...

Bah, sorry for the trouble :P