The 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.


First Experiment.
Second Experiment.
Third Experiment.
Fourth Experiment.

First Experiment:


The object/ball will roll down with increasing speed. It begins at rest, i.e., its speed is equal to zero, and then gradually gathers speed. The longer the inclined plane, the greater its speed. We call this increase in speed "acceleration." The opposite situation, in which a body gradually slows down, is known as "deceleration." Thus, we have seen that a body moving down an inclined plane accelerates downward.

What will happen to the ball after we give it an upward push?

The speed of a ball rolling up an inclined plane will gradually decrease, while that of a ball rolling down will gradually increase.

Second Experiment :

Is there a relationship between the plane's steepness and the acceleration of a body moving along this plane?

Between the angle of the plane's inclination and the change in the body's speed?

Third Experiment:

Is there a difference between a bicycle ride along a moderate incline and a steep incline?


Fourth Experiment:

What would happen if we were to place a moving frictionless ball on a horizontal plane, - i.e., a plane inclined neither downward nor upward?

  1. Will it increase its speed - accelerate?
  2. Will it decrease its speed - decelerate?
  3. Will it preserve its present speed?
Conclusions of The Third Experiment:

The more moderate a plane, the slower the acceleration of a body along it: i.e., the body's speed will increase at a slower rate up the plane. The more moderate the incline, the lower the deceleration of the body and the greater the distance it traverses.

We know that the force of gravity pulls heavy objects downward, toward the center of the earth. For this reason, bodies on an inclined plane are drawn downward. The smaller the incline, the lower the acceleration of the bodies moving down the plane and the lower the deceleration of objects moving up the plane.

Conclusions of The Fourth Experiment:

Is there something pulling the ball to the left or the right? The force of gravity? Any other force? Will a ball placed on the floor move by itself without being pushed?

If the plane had been somewhat inclined counter to the direction of the ball's movement, the ball would have decelerated; if the plane had been inclined in the direction of the ball's movement, it would have accelerated. Therefore, when the plane is not inclined at all (i.e., it is a horizontal plane), the ball will neither accelerate nor decelerate, but will preserve its present velocity.

A body moving on a smooth and frictionless horizontal plane will neither accelerate nor decelerate, but will continue to move at a constant speed. Such a body will only stop when another force stops it. This is in fact the law of inertia formulated clearly by Sir Isaac Newton in 1687, more than eighty years after Galileo began to investigate this law.

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