The Principle of Inertia

Physical effects relate only to a change in velocity

Consider two statements of a kind that was at one time made with the utmost seriousness:

People like Galileo and Copernicus who say Earth is rotating must be crazy. We know Earth cannot be moving. Why, if Earth was turning once every day, then our whole city would have to be moving hundreds of leagues in an hour. That is impossible! Buildings would shake on their foundations. Gale-force winds would knock us over. Trees would fall down. The Mediterranean would come sweeping across the east coasts of Spain and Italy. And furthermore, what force would be making the world turn?

All this talk of passenger trains moving at forty miles an hour is sheer hogwash! At that speed, the air in a passenger compartment would all be forced against the back wall. People in the front of the car would suffocate, and people at the back would die because in such concentrated air, they would not be able to expel a breath.

Some of the effects predicted in the first quote are clearly just based on a lack of experience with rapid motion that is smooth and free of vibration. But there is a deeper principle involved. In each case, the speaker is assuming the mere fact of motion must have dramatic physical effects. More subtly, they also believe a force is needed to keep an object in motion: the first person thinks a force would be needed to maintain Earth’s rotation, and the second apparently thinks of the rear wall as pushing on the air to keep it moving.

Common modern knowledge and experience tells us these people’s predictions must have somehow been based on incorrect reasoning, but it is not immediately obvious where the fundamental flaw lies. It is one of those things a four-year-old could infuriate you by demanding a clear explanation of. One way of getting at the fundamental principle involved is to consider how the modern concept of the universe differs from the popular conception at the time of the Italian Renaissance. To us, the word “earth” implies a planet, one of the nine planets of our solar system, a small ball of rock and dirt that is of no significance to anyone in the universe except for members of our species, who happen to live on it. To Galileo’s contemporaries, however, Earth was the biggest, most solid, most important thing in all of creation, not to be compared with the wandering lights in the sky known as planets. To us, Earth is just another object, and when we talk loosely about “how fast” an object such as a car “is going,” we mean the car-object’s velocity relative to the earth-object.

Motion is relative

According to our modern worldview, it is not reasonable to expect that a special force should be required to make the air in the train have a certain velocity relative to our planet. After all, the “moving” air in the “moving” train might just happen to have zero velocity relative to some other planet we do not even know about. Aristotle claimed that things “naturally” wanted to be at rest, lying on the surface of Earth. But experiment after experiment has shown there is really nothing so special about being at rest relative to Earth. For instance, if a mattress falls out of the back of a truck on the freeway, the reason it rapidly comes to rest with respect to the planet is simply because of friction forces exerted by the asphalt, which happens to be attached to the planet.

Galileo’s insights are summarized as follows:

The principle of inertia

No force is required to maintain motion with constant velocity in a straight line, and absolute motion does not cause any observable physical effects.

There are many examples of situations that seem to disprove the principle of inertia, but these all result from forgetting that friction is a force. For instance, it seems a force is needed to keep a sailboat in motion. If the wind stops, the sailboat stops too. But the wind’s force is not the only force on the boat; there is also a frictional force from the water. If the sailboat is cruising and the wind suddenly disappears, the backward frictional force still exists, and since it is no longer being counteracted by the wind’s forward force, the boat stops. To disprove the principle of inertia, we would have to find an example where a moving object slowed down even though no forces whatsoever were acting on it. Over the years since Galileo’s lifetime, physicists have done more and more precise experiments to search for such a counterexample, but the results have always been negative.


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