In 1687, Isaac Newton published his three laws of motion for the first time in his book Philosophiae Naturalis Principia Mathematica. These mathematical principles have withstood the test of time—and have become the foundation for a lot of further scientific investigation. See TeacherTech for a fun and easy-to-understand explanation of the laws.
Newton's first law is often called the law of inertia. It asserts, "A body continues to maintain its state of rest or of uniform motion unless acted upon by an external unbalanced force." In other words, an object will not start or stop moving unless something else causes it to do so. Or, as you might have heard in high school: Objects at rest stay at rest, and those in motion stay in motion, unless acted upon by an outside force.
The second law states that when F is the force applied to an object, m is the object's mass, and a is its acceleration, then F = ma. The greater the force applied to an object and the lower the object's mass, the faster it will accelerate. In other words, it takes less force to move an object with a lower mass than an object with a higher mass.
The most well known of Newton's laws of motion is the third one. It is usually stated, "To every action there is an equal and opposite reaction." Every time something moves, there is an equal force in the opposite direction acting on the object that caused the motion.