Before we get into the subject of gravity
and how it acts, it's important to understand the difference between
weight and mass.
We often use the terms "mass" and "weight" interchangeably in our daily
speech, but to an astronomer or a physicist they are completely different
things. The mass of a body is a measure of how much matter it contains. An
object with mass has a quality called inertia. If you shake an object
like a stone in your hand, you would notice that it takes a push to get it
moving, and another push to stop it again. If the stone is at rest, it wants to
remain at rest. Once you've got it moving, it wants to stay moving. This quality
or "sluggishness" of matter is its inertia. Mass is a measure of how much
inertia an object displays.
Weight is an entirely different thing. Every object in the universe with mass
attracts every other object with mass. The amount of attraction depends on the
size of the masses and how far apart they are. For everyday-sized objects, this
gravitational pull is vanishingly small, but the pull between a very large
object, like the Earth, and another object, like you, can be easily measured.
How? All you have to do is stand on a scale! Scales measure the force of
attraction between you and the Earth. This force of attraction between you and
the Earth (or any other planet) is called your weight.
If you are in a spaceship far between the stars and you put a scale
underneath you, the scale would read zero. Your weight is zero. You are
weightless. There is an anvil floating next to you. It's also weightless. Are
you or the anvil mass-less? Absolutely not. If you grabbed the anvil and tried
to shake it, you would have to push it to get it going and pull it to get it to
stop. It still has inertia, and hence mass, yet it has no weight. See the
difference?
The Relationship Between Gravity and Mass and Distance
As stated above,
your weight is a measure of the pull of gravity between you and the body you are
standing on. This force of gravity depends on a few things. First, it depends on
your mass and the mass of the planet you are standing on. If you double your
mass, gravity pulls on you twice as hard. If the planet you are standing on is
twice as massive, gravity also pulls on you twice as hard. On the other hand,
the farther you are from the center of the planet, the weaker the pull between
the planet and your body. The force gets weaker quite rapidly. If you double
your distance from the planet, the force is one-fourth. If you triple your
separation, the force drops by one-ninth. Ten times the distance, one-hundredth
the force. See the pattern? The force drops off with the square of the
distance. If we put this into an equation it would look like this:
