How Gravity Affects Motion
Gravity attracts all objects towards each other. Gravity has been around since the very beginning of the universe, and it works the same way everywhere in the universe, on all kinds of different objects, of all different sizes. The bigger an object is, and the closer you are to it, the stronger its gravitational pull is. On Earth, gravity keeps the air around us (and everything else) from drifting off into space. Gravity also causes things to fall to the ground, and causes the ocean's tides, and causes hot air to rise while colder air falls (which in turn causes wind).
Weight is the gravitational force exerted on an object.
Friction is a force that acts against or stops motion when two surfaces rub against each other.