Playing hockey, driving a car, and even simply taking a walk are all everyday examples of Newton's laws of motion. Compiled in 1687 by English mathematician Isaac Newton, the three main laws describe forces and motion for objects on Earth and throughout the universe.
Development of Classical Physics
Philosophers have studied the movement of objects since ancient times. After observing the motion of the sun, stars and planets, Greek philosopher Aristotle and later Ptolemy believed the Earth was the at the center of the universe. In 16th century Europe, Polish mathematician Nicolas Copernicus challenged this theory placing the sun at the center of the solar system with planets orbiting around it. The following century, German physicist Johannes Kepler described the elliptical orbits of planets, and Italian mathematician and astronomer Galileo Galilei conducted experiments to study the motions of projectiles. Isaac Newton synthesized this work into a mathematical analysis and introduced the concept of force and his three laws of motion.
First Law: Inertia
Newton's first law, also called the law of inertia, states that an object remains at rest or continues in uniform motion unless it is compelled to change by the action of an external force. The object’s tendency to remain at rest or maintain a constant speed is called inertia and its resistance to deviation from inertia varies with its mass. It takes physical effort -- a force -- to overcome inertia for a person to get out of bed in the morning. A bicycle or car will keep moving unless the rider or driver applies a frictional force through the brakes to stop it. A driver or passenger in a moving car who is not wearing a seat belt will be thrown forward when the car suddenly stops because he remains in motion. A fastened seat belt provides a restraining force on the passenger’s or driver’s motion.
Second Law: Force and Acceleration
Newton’s second law defines the relationship between the change in the speed of a moving object -- its acceleration -- and the force acting upon it. This force equals the object’s mass multiplied by its acceleration. It takes a smaller extra force to propel a small yacht at sea than to propel a supertanker because the latter has a greater mass than the former.
Third Law: Action and Reaction
Newton’s third law states that there are no isolated forces. For each force that exists, one of equal magnitude and opposite direction acts against it: action and reaction. For example, a ball thrown onto the ground exerts a downward force; in response, the ground exerts an upward force on the ball and it bounces. A person is unable to walk on the ground without the ground’s frictional force. When he takes one step forward, he exerts a backward force on the ground. The ground responds by exerting a frictional force in the opposite direction allowing the walker to move forward as he takes a further step with his other leg.