To explain this difference, let us take a heavy object with us into space, a 20-ton truck, for example. Once we have gone beyond the reach of the pull of gravity, this once extremely heavy object becomes weightless, but it still has the same mass as it had when it was on the surface of the Earth, and it still has the same inertia, too. So, mass has a great deal more to do with inertia than with weight; the more mass an object has, the more inertia it has, being directly proportional. And...what is inertia?
Inertia is the natural tendency of all object to keep the state of rest, or the state of movement, in which it is found, exerting resistance to change that state. Thus, the more mass an object has, the more resistance it will exert to being either moved or stopped, even if it is floating about weightlessly in outer space. And weight is simply the "pull downward" effect that a big-mass object's gravity exerts on another object of smaller mass. And the more mass an object has the more intense its gravity will be out there in space.
Thus, weight has something more to do with gravity, which is the mass effect. The Earth has more mass than the moon, so its pull of gravity is stronger that its satellite (we are lighter on the surface of the moon than when we are on Earth); and the Sun has a lot more mass than the Earth, so it is the Sun that exerts a gravitational pull on Earth and not the other way around.
Have I made myself clear? I hope so.
By Carlos B Camacho