# Homework Help: Gravity and circular motion

1. Mar 6, 2006

### rachael

q1. what is the acceleration of the moon towards the earth?
q2. Two satellites,X and Y, of equal mass are orbiting the Earth in orbits of radii r and 3r respectively. Find the ratios:
a. weight of satellite X/ weigh of satellite Y

thank you

2. Mar 6, 2006

### xman

a1: i'm assuming you're modeling the orbits as circular motion, either way, try writing the forces acting on the two bodies (earth and the moon), and draw a figure that should help.

a2: similar to q1, except you have to do it twice, one for each satellite. now you'll have two equations and with the relationships of the radii given, you may solve for the masses.
hope this helps, sincerely, x

3. Mar 6, 2006

### rachael

i dont get it

4. Mar 6, 2006

### HallsofIvy

[tex]F_g= -\frac{GMm}{r^2}[/itex]
where G is the universal gravitational constant, M and m are the masses of the objects and r is the distance between their centers.

5. Aug 22, 2009

### Jacky817

I don't think it is F=-(GMm)/r²
there should'nt be a minus sign there.
You might have confused it with gravitational potential energy? U=-(GMm)/r

6. Aug 22, 2009

### mg0stisha

no, HallsofIvy's equation is correct.

7. Aug 22, 2009

### Jacky817

my bad, without the minus sign is only a convention haha