Newton's second law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The principle stating that a force acting on a body is equal to the acceleration of that body times its mass, assuming a constant mass. Expressed mathematically, F = ma, where F is the force in Newtons, m is the mass of the body in kilograms, and a is the acceleration in meters per second per second.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics Newton's observation that the rate of change of the momentum of a body is directly proportional to, and in the same direction as, the net force acting on it.