Newton realized that gravity acts everywhere in the universe, not just on Earth. It is the force that makes an apple fall to the ground. It is the force that keeps all the planets in our solar system orbiting around the sun.

What Newton realized is now called the law of universal gravitation. The law of universal gravitation states that the force of gravity acts between all objects in the universe. This means that any two objects in the universe, without exception, attract each other. You are attracted not only to Earth but also to all the other objects around you. Earth and the objects around you are attracted to you as well. However, you do not notice the attraction among objects because these forces are small compared to the force of Earth's attraction.

Factors Affecting Gravity

Two factors affect the gravitational attraction between objects: Mass and Distance. Mass is a measure of the amount of matter in an object. The SI unit of mass is the kilogram. One kilogram is the mass of about 400 modern pennies. Everything that has mass is made up of matter.

The more mass an object has, the greater its gravitational force. Because the sun’s mass is so great, it exerts a large gravitational force on the planets. That’s one reason why the planets orbit the sun.

In addition to mass, gravitational force depends on the distance between the objects. The farther apart two objects are, the lesser the gravitational force between them. For a spacecraft traveling toward Mars, Earth’s gravitational pull decreases as the spacecraft's distance from Earth increases. Eventually the gravitational pull of Mars becomes greater than Earth’s, and the spacecraft is more attracted toward Mars.