Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Summary - Newton's Laws of Motion

Newton's first law of motion states that if all the forces on an object cancel each other out, then the object continues in the same state of motion. This is essentially a more refined version of Galileo's principle of inertia, which did not refer to a numerical scale of force.

Newton's second law of motion allows the prediction of an object's acceleration given its mass and the total force on it, acm = Ftotal/m. This is only the one-dimensional version of the law; the full-three dimensional treatment will come in the chapter on vectors. Without the vector techniques, we can still say that the situation remains unchanged by including an additional set of vectors that cancel among themselves, even if they are not in the direction of motion.

Newton's laws of motion are only true in frames of reference that are not accelerating, known as inertial frames.

Exploring Further

Isaac Newton: The Last Sorcerer, Michael White. An excellent biography of Newton that brings us closer to the real man.

Last Update: 2009-06-21