The term laws of motion generally refers to three statements originally devised by English physicist Isaac Newton (1642–1727) in the 1680s. These laws, along with Newton's law of gravitation, are generally considered to be the ultimate solution to a problem that had troubled scholars for more than 2,000 years: motion.
Examples of motion are everywhere in the world around us. What makes a rock fall off a cliff? How does a skate slide across an icy surface? What keeps the planets in their orbits around the Sun? It is only natural, then, that questions about motion were foremost in the minds of ancient philosophers and physicists.
Greek philosopher Aristotle (384–322 B.C. ), for example, tried to find the causes of motion. He said that some forms of motion were "natural." Rocks fall toward the ground because the ground is a natural place for rocks to be. Objects rise into the air when they are heated because the Sun is hot, and so it is natural for heat to rise.
Aristotle classified other forms of motion as "violent" because they were not natural to his way of thinking. For example, shooting an arrow through space produced violent motion since the arrow's natural tendency was to fall straight down toward Earth.
Aristotle's thinking about motion dominated Western thought for 2,000 years. Unfortunately, his ideas were not really very productive, and scholars tried continually to improve on the concepts of natural and violent motion—without much success.
Then, in the early seventeenth century, Italian astronomer and physicist Galileo Galilei (1564–1642) proposed a whole new way of looking at the problem of motion. Since asking why things move had not been very productive, Galileo said, perhaps physicists should focus simply on describing how they move. A whole new philosophy of physics (the science of matter and energy) was created and, in the process, the science of physics itself was born.
Newton, who was born in the year that Galileo died, produced a nearly perfect (for the time) response to Galileo's suggestion. He said that the movement of objects can be fully described in only three laws. These laws all show how motion is related to forces. One definition for the term force in science is a push or a pull. If you push a wooden block across the top of a table, for example, you exert a force on the block. One benefit of Newton's laws is that they provide an even more precise definition for force, as will be demonstrated later.
The first law. Newton's first law of motion is that an object tends to continue in its motion at a constant velocity until and unless an outside force acts on it. The term velocity refers both to the speed and the direction in which an object is moving.
For example, suppose that you shoot an arrow into space. Newton's first law says that the arrow will continue moving in the direction you aimed it at its original speed until and unless some outside force acts on it. The main outside forces acting on an arrow are friction from air and gravity.
Acceleration: The rate at which the velocity of an object changes with time.
Force: A physical interaction (pushing or pulling) tending to change the state of motion (velocity) of an object.
Inertia: The tendency of an object to continue in its state of motion.
Mass: A measure of an amount of matter.
Velocity: The rate at which the position of an object changes with time, including both the speed and the direction.
As the arrow continues to move, it will slow down. The arrow is passing through air, whose molecules rub against the arrow, causing it to lose speed. In addition, the arrow begins to change direction, moving toward Earth because of gravitational forces. If you could imagine shooting an arrow into the near-perfect vacuum of outer space, the arrow would continue moving in the same direction at the same speed forever. With no air present—and beyond the range of Earth's gravitational attraction—the arrow's motion would not change.
The first law also applies to objects at rest. An object at rest is simply an object whose velocity is zero. The object will continue to remain at rest until and unless a force acts on it. For example, a person might hit the object with a mallet. The force of the blow might change the object's motion, giving it both speed and direction.
The property of objects described by the first law is known as inertia. The term inertia simply means that objects tend to continue in whatever their state of motion is. If moving, they continue to move in the same way, or, if at rest, they continue to remain at rest unless acted on by an outside force.
The second law. Newton's second law clearly states the relationship between motion and force. Mathematically, the law can be stated as F = m · a , where F represents the force exerted on an object, m is the object's mass, and a is the acceleration given to the object. The term acceleration means how fast the velocity of an object is changing and in what direction.
To understand the second law, think of a soccer ball sitting on the ground. If you kick that ball with a certain force, the ball will be given a certain acceleration. If you kick the ball with twice the force, the ball will be given twice the acceleration. If the ball then bounces off the goal post and out of bounds, the force of the impact with the goal post will change the ball's direction.
The second law provides a more precise way of defining force. Force is any action that causes a body to change the speed or direction with which it is moving.
The third law. Newton's third law says that for every action there is an equal and opposite reaction. A simple example of the law is a rocket. A rocket is simply a cylindrical device closed at one end and open at the other end in which a fuel is burned. As the fuel burns, hot gases are formed and released through the open end of the rocket. The escape of the gases in one direction can be considered as an action. Newton's law says that this action must be balanced by a second action that is equal in magnitude and opposite in direction. That opposite action is the movement of the rocket in a direction opposite that of the escaping gases. That is, the gases go out the back of the rocket (the action), while the rocket itself moves forward (the reaction).