VI.

Isaac Newton

Newton is considered the greatest physicist of all time, with the debatable exception of German-born American physicist Albert Einstein. Newton carried Galileo’s study of motion forward in a major way. Galileo carried out experiments and described mathematically how objects moved, but he did not address the issue of why they moved. For example, Galileo described the distance an object will fall after a particular amount of time has passed by the equation x = 1/2at².

The x represents the distance moved, and t is the time the object has been falling. The a in the equation stands for the acceleration of the object, which Galileo had defined as a measure of how velocity changes. This quantity depends on conditions. For example, a ball dropped from a tower falls with a particular value of a, whereas a ball sliding down an inclined plane has a much smaller value of a, depending on the angle of the tilt of the plane. But why?

Newton carried Galileo’s equation forward by introducing the concept of force. He began by restating Galileo’s law of inertia as a situation applying to the absence of force. Newton’s first law states “A body in motion remains in motion (with constant velocity) unless acted upon by an external force.” Rest, in Newton’s laws, is merely an example of motion with zero velocity. So force is defined as the agency that changes the state of motion, and thus the velocity, of a body.

Newton’s famous second law relates a to a force acting on the object via the equation F = ma. The quantity m is the “stuff” inside the object, which Newton called inertial mass. The bigger the value of m, the larger the force required to get the object moving—that is, accelerating. Applied to Galileo’s experiments, F is the force of gravity tugging at the object and aimed toward the center of Earth. F is carefully defined as the sum of all forces.

Forces can be exerted by a variety of things—air, surfaces, walls, or hammers. But forces can also be exerted by electric fields and by magnetic fields. Many scientists who explored nature after Newton applied his equation to their own studies of electric and magnetic fields.

Newton’s second law accounted for the motion of planets pulled by the Sun’s gravitational force; the motion of projectiles, influenced by air and the pull of gravity; and the tides, which are caused by ocean waters pulled by the Sun and the Moon. Newton proved mathematically what Kepler had concluded from observations—that planets move in elliptical paths. To make this proof, he had to know the precise form of F. F must change, depending on the distance of the planet from the Sun. So Newton had to guess the way the force of the Sun on a planet grows weaker as the distance between these two objects increases. His guess was an inverse square law, which states that the force of gravity is inversely proportional to the square of the distance between the two objects.

Newton’s equations also took into account the fact that objects have two kinds of masses: inertial mass that resists motion and gravitational mass that encourages motion. He wrote another equation illustrating that for any object, whether it be a steel ball, a wood block, water, or a planet, the two types of masses—gravitational mass and inertial mass—are equal. Einstein would return to this idea in his general theory of relativity, wherein he made the equality of inertia and gravitational masses a key point.

This equality accounts for many curious results. For example, Galileo had discovered that heavy objects and light objects fall with the same speeds. Aristotle was sure that heavy objects would fall faster, although he never tried it. We now can see why Galileo’s discovery is true. The Earth pulls harder on a heavy thing, but the inertial mass of a heavy thing is higher; the two effects cancel out, and all bodies fall with the same speed. Of course, the influence of air complicates the experiment.

The curious behavior of objects in space capsules, which we call weightlessness, works on the same principle. The astronaut, his sandwich, and his drink all float together, apparently without gravity. But gravity is still pulling on the astronaut, and the capsule, and the sandwich. They respond according to their inertia, and the two effects cancel each other out.

Newton’s work was vital to the evolution of modern physics. In fact, many of his theories and conclusions remained free of revisions until the 20th century.