# Homework Help: Need universal Gravitation help

1. Jul 13, 2011

### Combine

1. The problem statement, all variables and given/known data
what is the magnitude of Gravitational force
for the following picture(s)
(attached to forum)

2. Relevant equations
Fg = G M*M/D^2

3. The attempt at a solution
I tried to do some of them I know the equation for the second one is fG = 2G M*M/D^2
and for the third one it's Fg = 6G M*M/D^2
But I have no idea where the 2 came from for the second one and the 6 from the third one!

#### Attached Files:

• ###### Questions.png
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2. Jul 13, 2011

### Staff: Mentor

A better way to write that force is F = G M1 M2 / D^2, where M1 and M2 are the two masses. Does that help?

3. Jul 13, 2011

### Combine

Kinda, I still don't understand where the 2 in fG = 2G M1 * M2/D^2
or the 6 in fG = 6G M1 * M2/D^2 come from

4. Jul 13, 2011

### Staff: Mentor

In the 3rd one, what are M1 and M2?

5. Jul 13, 2011

### Combine

According to my sheet, it's just simply m1 and m2, there are no mass representing them, I think you just have to write the final equation

6. Jul 13, 2011

### Staff: Mentor

There are two masses and a distance indicated. Call the mass on the left M1, and the mass on the right M2. What are M1 and M2 according to the figure? What is the distance?

#### Attached Files:

• ###### Fig1.gif
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7. Jul 13, 2011

### Combine

there is no mass specified it just says M1 and M2 and distance is just represented by d

8. Jul 13, 2011

### Staff: Mentor

Look more closely at the diagram. I don't see the characters "M1" or "M2" in the figure. How are the masses labelled?

9. Jul 13, 2011

### Combine

Never mind my teacher answered the question for me (after several rather painful hours of waiting) we just have to make the equation nothing special, Fg = 6G M1 * M2 / d
and he also explained to me what the 6 was...

I am such an idiot >.<
thanks for the help though

10. Jul 13, 2011

### TheDuncster94

You just insert the numbers in the pictures into the equation. For example:

The second one - $F=\frac{G2mM}{{\Delta}d^{2}}$

Which also is equal to - $F=\frac{2GmM}{{\Delta}d^{2}}$

Basically, you take the number of the specific mass or ${\Delta}d$ and multiply it or divide it by the appropriate amount as represented by the picture.

The third one is $F=\frac{G2m_{1}3m_{2}}{{\Delta}d^{2}}$ and it simplifies to $F=\frac{6Gm_{1}m_{2}}{{\Delta}d^{2}}$

EDIT: Ah whoops, I replied to late...