Overview
Isaac Newton is considered by many to be the father of modern physics. He invented calculus, and with calculus in hand, people could now create mathematical models of changing systems. His three laws of motion and his law of gravitation, however, are still one of the first things that all physics students learn, and are the basis behind a lot of physics that is done today.
Although Newton made many very important contributions to the world of science, the most important to astronomy were his laws of motion and law of gravity.
Laws of Motion
Newton's Laws of Motion are still used by physicists all over the world. Newtonian Physics is taught in many courses at the college level, as well as high school and middle school. Everything in that genre of physics is based upon these three laws:
Every object has uniform motion unless acted upon by a force.
The force applied to an object is equal to the object's mass times the resulting acceleration:
For every action, there is an equal and opposite reaction.
These laws are used to describe everything from throwing a ball to the merging of galaxies. Even though it has been shown through people like Albert Einstein that Newton's Laws are really approximations, they work so well on a wide range of scales, that they are still used in lieu of the better laws. For example, you would use Newtonian Mechanics to describe the acceleration of a car, not Einstein's General Relativity.
Law of Gravity
Newton's Law of Gravity is not precise in extreme circumstances, such as very high velocities or very strong gravity. For cases such as these, Einstein's General and Special Relativity theories are needed. However, in most other cases, and especially those that we are familiar with on Earth, Newton's Law works extremely well.
It is based upon his laws of motion, and it shows how two objects exhibit a force upon the other. It is the equation to the right.
It says that the gravitational force experienced is equal to a gravitational constant times both masses divided by the distance between them squared. The value "G" is an extremely small number, and therefore the gravitational force is extremely weak - the weakest of the four fundamental forces. This law also shows that the force of gravity dies off with the square of the distance. This means that if you are twice as far away from something, then the gravitational force you experience is 1/4 as much.
