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3.10 Isaac Newton

In the 1600s, scientists hammered out the method by which modern scientists approach a problem. Kepler deduced that elliptical orbits were a good description or model of the motions of the planets. He also found patterns in the motions and spacing of the planets that supported the Copernican hypothesis. Astronomers now needed a more complete physical theory to explain and predict the observed phenomenon. To be successful, the theory would have to start with a few universal principles and show that Kepler's elliptical orbits were a consequence of these principles. If such a theory could be developed, scientists believed they would be able to understand other phenomena in nature as well.

The man who achieved the synthesis that explained planetary motions — a man usually considered the greatest and most creative physicist who ever lived — was Isaac Newton. Newton revolutionized the fields of physics and astronomy. He was born in the south of England in 1643, just after the death of Galileo. He was a lonely and moody boy, often preoccupied by his own thoughts. Between the ages of 23 and 25, while attending Cambridge, he single-handedly developed calculus, discovered the principle of gravitational attraction and certain properties of light, and improved the design of the telescope. Newton once said that he made his discoveries "by always thinking about them without ceasing," a trait that no doubt contributed to his reputation for absent-mindedness.

Newton thought deeply about the way objects move and came up with three laws of motion that describe the mechanics of objects in the everyday world. The first stemmed from Galileo's ideas about inertia: a body at rest stays at rest or moves at constant speed in a straight line unless an unbalanced force acts on it. There are several important ideas here. The first is that uniform motion is just as natural as no motion, which is a major departure from Greek physics. The second is that an object changes its motion because a force is acting — this is the way we define the concept of a force. For example, you know that a rolling ball always comes to a halt. Newton realized that it slows down because the force of friction is acting. In a situation with less friction, such as a hockey puck sliding across ice, the motion would be nearly uniform. What do we mean by "unbalanced force?" When you sit on a chair you do not move because the chair pushes up on you with the same force that gravity pulls down on you. If it did not, the chair would break and your stationary state would suddenly change!

Oblique view of the phases of Venus. Click here for original source URL.

Newton’s second law mathematically relates a force to the change in motion it causes. For every force, there is a corresponding acceleration, which is proportional to the force and is in the direction of the force, and inversely proportional to the mass of the body. In other words, force equals mass times acceleration. The key concept here is mass, which is a measure of inertia or the resistance of any object to a change in its motion. If you push hard on a shopping cart it gives little resistance to the force so you can change its motion or accelerate it. A similar push on car (with the brake off) would produce little change in its motion. For any object, a larger force produces a larger acceleration.

Newton’s third law is sometimes called the principle of action and reaction. It states that for every force (sometimes called an action) there is an equal and opposite force (called a reaction). We do not see single isolated forces in nature. There is always an opposite-directed force as well. Take the case of rocket propulsion, for example. Rocket fuel creates a force that accelerates the rocket forward, but an equal force ejects exhaust gas at very high speed backward. If you foolishly punch a wall, the force you exert on the wall is countered by an equal force on your hand that will cause much pain. The same principle occurs in the recoil of a gun. Or consider the fact that when you step off a small boat onto a pier, the force that gets you onto the pier is countered by a force that sends the boat away from the pier. Sometimes the reaction force is concealed by the very different masses of the two objects. When you jump into the air, you do exert a downward force on the entire Earth, but the mass and inertia of the Earth are so large that you cannot perceptibly change its motion in this way.

Newton’s understanding of the way forces act in the universe was so broad that the unit of force is named after him. As with most other physical concepts, force can be described in terms of the three fundamental units: mass, length, and time. Many apparently complex parameters can be reduced to combinations of these three units. Newton’s laws of motion are the basis of the subject of mechanics; the study of the way objects move. They can be summarized as follows:

• An object stays at rest or in constant motion unless an unbalanced force acts on it.

• An object responds to a force with an acceleration that is proportional to the force and in the direction of the force, and inversely proportional to the mass.

• For every force on an object, there is an equal and opposite reaction.

Newton’s laws of motion have a profound implication for how science works. Science depends on the assumption of causality — that every effect has an identifiable cause. Aristotle was forced to explain the different motions of a falling feather and a falling rock in terms of their different "natures." Newton removed all this unexplained mystery by identifying causes for motions of all kinds. A rolling ball stops because the force of friction is acting. The bird keeps flying because its beating wings exert an upward force in the air. Equally, nature is not capricious. Fallen objects do not spontaneously right themselves. Newton’s laws establish that the universe is a rational place where events have causes.

Newton’s work had a major practical consequence. The systematic understanding of motions and forces led clever inventors to try and harness those forces in useful machines. The spinning "jenny" and the steam engine were the first examples in a sequence of innovation that would transform the economic landscape of England and eventually the world. Within a hundred years of Newton's work the Industrial Revolution was underway.