There can be no quarrel—an inertial frame of reference is convenient and has invaluable advantages.

But is such a frame of reference unique or do there, perhaps, exist many inertial frames of reference? The Ancient Greeks, for example, took the former point of view. In their writings we find many naïve reflections on the causes of motion. These ideas find their completion in Aristotle. In the opinion of this philosopher, the natural state of a body is rest—of course, with respect to the Earth. Every displacement of a body with respect to the Earth must have a cause—a force. But if there is nothing causing a body to keep moving, it must halt, return to its natural state. And this is what rest with respect to the Earth is. From this point of view, the Earth is the unique inertial frame of reference.

We are indebted to the great Italian Galileo Galilei (1564-1642) for discovering the truth and disproving this false but congenial to naive psychology opinion.

Let us think over the Aristotelian explanation of motion and search familiar phenomena for confirmation or refutation of the idea that rest is the natural state of bodies on the Earth.

Imagine that we are in an airplane taking off from an airport at dawn. The Sun has not yet warmed up the air, so there are no “air-pockets”, which cause many passengers unpleasantness. The airplane is moving smoothly, imperceptibly. If you don’t look out of the window, you won’t even notice that you’re flying. A book is lying on an empty seat; an apple is at rest on a table. All objects inside the airplane are motionless. Is this how things should be if Aristotle were right? Of course not. As a matter of fact, according to Aristotle, the natural state of a body is rest on the Earth. But then why are all the objects not piled up at the rear wall of the airplane trying to lag behind its motion, “wanting” to return to the state of “true” rest? What makes the apple lying on the table, hardly touching the surface of the table, move with an enormous speed of several hundred kilometres an hour?

What is the correct answer to the question of the cause of motion? Let us first take up the question of why moving bodies come to a stop. For example, why does a ball rolling along the Earth’s surface come to a stop? In order to give a correct answer, we should consider in which cases a ball comes to a stop quickly, and in which cases slowly. We don’t need any special experiments for this. We know perfectly well from our practical experience that the smoother the surface on which a ball is moving, the farther it will roll. From these and similar experiences, there arises the natural idea of the force of friction as a hindrance to motion, as the cause for the slowing down of an object which is rolling or slipping’ along the Earth. Friction can be decreased in various ways. The more we work on the destruction of every kind of resistance to motion (for example, the smoother we construct our roads, the better we lubricate our engines and the more we perfect our ball bearings), the greater the distance a moving body will cover freely without being acted on by any external force.

The following question arises: What would happen if there were no resistance, if the force of friction were absent? Obviously, in such a case a motion would continue infinitely, with a constant speed and along one and the same straight line.

We have formulated the law of inertia in about the same form as it was first given by Galileo. Inertia is a brief designation for this ability of a body to move rectilinearly and uniformly without any cause, contrary to Aristotle. Inertia is an inalienable property of each particle in the Universe.

In what way can we check the validity of this remarkable law? As a matter of fact, it is impossible to create conditions under which no forces would be acting on a moving body. Even though this is true, we can, on the other hand, observe the opposite. In every case when a body changes the speed or direction of its motion, it is always possible to find a cause—a force responsible for this change.

A body acquires speed in falling to the Earth; the cause is the Earth’s gravitation. A stone twirls on a string circumscribing a circle; the cause deflecting the stone from a rectilinear path is the tension in the string. If the string breaks, the stone will fly off in the same direction in which it was moving at the moment the string broke. An automobile running with the motor turned off slows down; the causes are air resistance, friction between the tires and the road, and imperfections in the ball bearings.

The law of inertia is the foundation on which the entire study of the motion of bodies rests.

Galileo Galilei [1564–1641]—a great Italian physicist and astronomer, the first to apply the experimental method of investigation in science. Galileo introduced the concept of inertia, established the relativity of motion, investigated the laws of free fall, of the motion of bodies on an inclined plane, and of the motion of an object thrown at an angle to the horizontal, used a pendulum for the measurement of time. For the first time in the history of mankind, he looked at the sky through a telescope, discovered many new stars, proved that the Milky Way consists of an enormous number of stars, discovered Jupiter’s satellites, sunspots and the rotation of the Sun, investigated the structure of the Moon’s surface. Galileo actively supported Copernicus’ heliocentric system banned in those days by the Catholic church. Persecution by the Inquisition darkened the last ten years of the great scientist’s life.