Forces - Newton's Laws

Newton's Laws were one of the first steps to describing motion more accurately. The first of Newton's Laws is known as The Law of Inertia. It states that an object at rest will stay at rest and an object in motion will continue to move provided there are no external forces on the object. This countered previous notion that an object must be pulled to maintain motion.
Newton's Second law states that force is proportional to mass and acceleration. This is where the definition of force is stated. In more mathematical terms,

F=m*a

where F is the force, m is the mass and a is the acceleration.
Newton's Third Law states that each force and an equal and opposite reaction. A specific example would be that if you throw a baseball, i. e. exerting a force on it. It will exert a force back on you; generally, the recoil from the ball.