# Newtons laws of universal gravitation

1. Mar 18, 2014

### Queen B

1. The problem statement, all variables and given/known data
its 3.8 x 10^8 from Earth's core to the lunar's core
calculate the gravitational attraction exerted by earth on the moon

2. Relevant equations
ag = GM/ r^2
G= 6.67 x 10^-11
M of earth= 5.98 x 10^24 ... r= 6.37 x 10^6
M of moon= 7.36 x 10^22 ... r= 1.74 X 10^6

3. The attempt at a solution
ag = ( 6.67 x 10^-11)( 7.36 x 10^22) / ( 6.37 x 10^6) ^2 = 0.12 N
I'm not sure if i'm right or not, I am just really confused as to where I plug in the numbers

2. Mar 18, 2014

### Andrew Mason

When the question asks for the gravitational attraction it is asking for the force. You are calculating the acceleration (toward the centre of mass of the earth-moon system). The units would m/sec^2.

Other than that your answer appears to be correct. Welcome to PF by the way!

AM

3. Mar 18, 2014

### Queen B

But how would I calculate the force? And thank you.

4. Mar 18, 2014

### SteamKing

Staff Emeritus
What is Newton's Second Law of Motion?

5. Mar 18, 2014

### Queen B

F=ma, ok i see now but what mass would i use?

Last edited: Mar 19, 2014
6. Mar 19, 2014

### SteamKing

Staff Emeritus
If you have calculated the acceleration of the moon caused by the earth, then you would use the mass of the ...?

7. Mar 19, 2014

### Queen B

The Earth?

8. Mar 19, 2014

### SteamKing

Staff Emeritus
Try again.

9. Mar 19, 2014

### Queen B

The Moon.

10. Mar 19, 2014

### SteamKing

Staff Emeritus
Good job.