The term laws of motion generally refers to three statements originally devised by English physicist Isaac Newton (1642–1727) in the 1680s that describe the relationships between objects and forces acting on them. These laws, along with Newton’s law of gravity, are the foundation of the branch of physics called mechanics.
Examples of motion are everywhere in the world. But it is not easy to explain why things move as they do. 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? Ancient scientists puzzled over these questions but could not come up with an adequate solution.
Greek philosopher Aristotle (384–322 BC), divided motion into two forms: natural and violent. 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. But shooting an arrow through space produces violent motion because the arrow’s natural tendency is to fall straight down toward Earth; shooting it disturbs its natural tendency. Aristotle’s ideas about motion dominated Western thought for 2,000 years.
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. Because asking why things move had not been very productive, Galileo said, perhaps physicists should focus simply on describing how things 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’s Three Laws
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. Pushing a wooden block across the top of a table, for example, exerts 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 states 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 an arrow is shot into space. Newton’s first law says that the arrow will continue moving in the direction it was aimed at its original speed until and unless some outside force acts on it. The main outside forces acting on an arrow are gravity and friction from air.
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 it were possible to shoot 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 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 that ball is kicked with a certain force, the ball will be given a certain acceleration. If the ball is kicked 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 an object 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).