THE LAW OF INERTIA

T he law of inertia states that:

A body will preserve its velocity and direction so long as no force in its motion's direction acts on it.

For example : a package thrown out of an airplane will continue to move at the speed of the airplane on the horizontal axis (in the direction of the airplane's movement). Since the law of gravity acts on the package (a vertical downward axis), the package will gather speed along the vertical axis, but on the horizontal axis its speed will remain equal to that of the airplane.

(In this explanation we have left out the force of air-resistance).

The law of inertia is the basis of the new physics of the seventeenth century. This law is also true according to modern physics. Galileo discovered the law during the first decade of the seventeenth century, but in fact he did not understand the law in the general way we have formulated it here. The general formulation of the law of inertia was devised by Galileo's pupils and by Descartes - a French philosopher, mathematician and physicist. This law is also the first of Newton's three laws .

Up to the time of Galileo, it was thought that one must exert force in order to cause and preserve motion, as claimed by the physics of Aristotle. Indeed, when we look at the world surrounding us, we see that in order to continue movement we must exert force. Thus, for example, in order to conserve the speed of a car, the engine must work. Objects on which no force is exerted to preserve their movement eventually come to a stop. Galileo understood that one can explain the stopping of bodies by the common experience that we always encounter a force of friction which resists the motion of bodies. However, without such resistance force, the bodies would continue to move at their previous speed.

The law of inertia is also important for Galileo's astronomy. He used this law to explain why we do not feel the earth's motion, and especially why objects falling on the surface of the earth move together with the earth. This explanation is related to the law of relativity. which is also based on the constant acceleration of bodies. In this way, Galileo succeeded in refuting the claims of his opponents, as in the example of the boat in which Galileo proves the law of inertia. Galileo suggested a number of additional proofs for this law with the help of the inclined plane. You will find an additional explanation next to the globe in the exhibition room.