So I was thinking about the equation for the acceleration of gravity on earth, or g: m*g=G*M(earth)*m/d^2 I guess it's universally accepted that different masses on earth's gravitational acceleration is 9.8m/s^2, regardless of the mass of the object. This is because the two "m"s cancel out, no? But do they HAVE to cancel out? I mean, if mass is supposed to be a playing factor for gravity universally, wouldn't on earth count too? That's like saying that me and another guy have a mass of 2*40kg=2*40kg, with my mass on the left and his on the right. Now, the two "2"s "can" cancel out, but if I did that, I would no longer have a mass of 80kg, I would only be 40kg, which simply isn't true. So doesn't canceling out the "m"s make the equation untrue as well? You don't have to necessarily cancel two variables out just because you can, because it could possibly make the equation "untrue", even if mathematically it's still correct, in reality it isn't "true".