# Homework Help: Net gravitational force on earth

1. Jul 20, 2011

### fa08ti

the earth has a mass of 5.98 x 10^24 kg and the moon has a mass of 7.35 x 10^22 kg. the distance from the centre of the moon to the centre of the earth is 3.84 x 10^8m. a rocket with a total mass of 1200kg is 3.0 x 10^8 m from the centre of the earth and directly in between the earth and the moon. find the net gravitational force on the rocket from the earth and moon.

ATTEMPT

ok so i used Fnet=( (Gm1m2)/r^2) + ( (Gm2m3)/r^2)

m1 would be the mass of earth, m2 is the mass of the rocket and m3 is the mass of the moon
what i'm confused about are the r values. for the first one i used 3.0 x 10^8 and for the second r value is subtracted 3.0 x 10^8 from 3.84 x 10^8m. is that correct?
i got a big number for the answer (2.30 x 10^9 N) so i want to make sure i understand how to do this

2. Jul 20, 2011

### Mike Pemulis

Very close; there is only one error. Hint: think about the directions of the forces.

3. Jul 20, 2011

### fa08ti

i was thinking i should subtract the values because the rocket is in the middle and since gravity attracts and doesn't repel. is that anywhere near logical?

4. Jul 21, 2011

### cupid.callin

Yes you are correct

5. Feb 26, 2012

"a rocket with a total mass of 1200kg is 3.0 x 10^8 m from the centre of the earth and directly in between the earth and the moon."

Doesn't this mean that the rocket forms a triangle with the earth and moon?
If so, would we not have to calculate the distance between the rocket and the moon, and then add the forces together to get the net gravitational force?

Or is the rocket directly in line with the earth and moon...

Last edited: Feb 26, 2012
6. Mar 4, 2012

### cupid.callin

" directly in between the earth and the moon " would imply that all the 3 are on a straight line.
And yes, we would have to calculate distance b/w rocket and moon, though it is not at all difficult i suppose :D

And then we would simply use Sir Issac Newton's Law Of Gravitation.