ok I'll be using a home work problem to ask the question. I'm not asking for help on the question I got the correct answer just need to understand this concept better. Calculate the acceelration due to gravity on the Moon. The moon's radius is about 1.74 E 6 meters and its mass is 7.35 E 22 kg. For this problem I'll be using m2 as the mass which the moon is orbiting Ok then... [tex]\Sigma[/tex]F [in radial direction acting on m2] = (m2 a [radial direction] = Fg = r^-2 G m m2)m2^-1 divide both sides by m2 a [radial direction] = r^-2 G m I rember in like inclined planes and such when two forces equal and opposite each other are present you can just cancel them out... so how is there any net force? how come like in inclined planes when I find the net force i don't include both masses... Net force = (net mass) a I dont use net mass in this problem because of why? Why do I just use m2 and not m1 + m2 net force = (m1 + m2) a ?????