The terms gravity and gravitation are often used to explain the same thing, but there is a definite difference between the two. Gravitation is the attractive force existing between any two objects that have mass. The force of gravitation pulls objects together. Gravity is the gravitational force that occurs between the Earth and other bodies. Gravity is the force acting to pull objects toward the Earth. Since gravitational force is happening to all objects in the universe, from the largest galaxies down to the smallest atoms, it is often called universal gravitation. Gravitation is actually a very weak force. The pull is too weak to be felt between two people and it is not even strong enough to pull together two lumps of lead placed right next to each other. It is only when one of the masses is the size of a planet that we can feel the force of gravity. The huge gravitational force of our nearest star, the Sun, holds together the eight planets of our Solar System. The planets move through space at speeds that just balance the Sun’s gravitational pull, so they are locked into a permanent orbit around the Sun. Moons orbit planets, and satellites and spacecraft orbit the Earth, in the same way. Satellites are not defying gravity in circling endlessly around the Earth, it is just that they are moving so fast around the Earth that gravity never brings them any closer. The force holding objects to the Earth's surface depends not only on the Earth's gravitational field but also on other factors, such as the Earth's rotation. The Earth’s gravitational pull extends out into space in all directions. The further we move away from the centre of the Earth the weaker the force become. The measure of the force of gravity on object is the weight of that object. The weight of an object changes depending on its location in the universe.
