NASA - National Aeronautics and Space Administration

When you think of Sir Isaac Newton, you might imagine a man sitting under an apple tree. It is said that Newton discovered gravity when an apple fell on his head and he wondered what made the apple fall down instead of float away or go up. In 1687, Newton published a book with his explanation of how an object, or matter, moves. The book, entitled "Principia," is important to science because in it he explained three laws of motion and the Universal Law of Gravity. Newton's Laws of Motion describe the relationships between motion, matter and force.

Law	Also known as ...	Description
First law of motion	Law of inertia	An object that is not moving will not move until a force makes it move. An object that is moving will continue to move at a constant speed and direction until a force causes it to change.
Second law of motion	F=ma	The force of an object equals its mass times its acceleration.
Third law of motion	Law of action and reaction	For every action there is an equal and opposite reaction.

Newton's second law is a formula. If force equals mass times acceleration, then acceleration equals force divided by mass. When the formula is written this way, it explains that an object's speed, or velocity, will depend on its mass and the force that is applied to it.

Newton's laws are important to NASA. They determine how NASA launches rockets, flies airplanes, and conducts tests and experiments. These laws also help in understanding other scientific principles, such as how planets orbit the sun. The laws of motion work in space and on Earth. Gravity is a strong force that we encounter on Earth. On the International Space Station, demonstrating these laws is easier because of microgravity.

More About Newton's Laws
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>  Sir Isaac Newton   →


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