**Astronomy — Newton’s Three Laws of Motion**

*The three laws of motion proposed by Sir Isaac Newton were these:*

1. A body continues at rest or in uniform motion in a straight line unless acted upon by some force.

2. A body’s change of motion is proportional to the force acting on it and is in the direction of the force.

3. When one body exerts a force on a second body, the second body must exert and equal and opposite force back on the first body.

He derived these laws from his study of the work of Galileo, Kepler, and others. His laws made it possible to predict exactly how a body would move if the forces were known. In thinking of the motion of the moon, Newton realized that some force had to pull the Moon toward Earth’s center. With no force to alter its motion, it would continue moving in a straight line and leave Earth forever. It could circle Earth only if Earth attracted it. Newton’s insight was to recognize that the force that held the Moon in its orbit was the same as the force that made apples fall from trees. The force of gravity was universal — that is, all objects attract all other objects with a force that depends only on their masses and the distance between their centers.

The mass of an object is a measure of the amount of matter in the object, usually expressed in kilograms. Mass is not the same as weight. An object’s width is the force that Earth’s gravity exerts on the object. Thus an object in space far from Earth might have no weight, but it would continue with the same amount of matter and would thus have the same mass that it would have on Earth.

With this definition of mass, we can describe Newton’s law of gravity in a simple equation: F = -G (Mm/*r*^{2}) where M and m are the masses of the two objects, *r* is the distance between their centers, G is the gravitational constant, and F is the force of gravity acting between the two objects.

In developing the 3 laws of motion and the theory of gravity, Newton resolved the problem Kepler had toyed with — why planets move along in their orbits. In fact, Newton’s work explained orbital motion and made it possible to analyze the motions of the heavens with great precision.

Click here to return to the Astronomy index page