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LESSON NOTES
THE LAW OF INERTIA

It was Galileo who first asserted that objects move with constant speed when no
external forces act on them. He arrived at this revolutionary conclusion on the basis of
following simple experiment

Galileo thought of an imaginary experiment. In this experiment, two inclined planes are
arranged facing each other, as shown in Fig.(a).

(i) When an object rolls down one of the inclined planes, it climbs up the other. It almost
reaches the same height but not completely because of the presence of friction. If the
friction were absent, the object must have reached the same height as the initial height,
as shown in Fig. (a).
(ii) When the slope of the upward inclined plane is decreased, the object has to travel a
longer distance to reach the maximum height, as shown in Fig. (b). The more we
decrease the slope of the upward inclined plane, the longer would be the distance that
the object is needed to travel to reach the same height.

Fig. Galileo's observations of motion on a double inclined plane.

(iii) From the above two observations, Galileo argued that if the second plane is made
horizontal [Fig(c)], the object will have to travel an infinite distance to reach the same
height. This is possible only if the object moves forever with uniform velocity on the
horizontal surface.

From the above series of experiments, Galileo formulated the following law of inertia :
A body moving with a certain speed along a straight path will continue to move with
same speed along the same straight path in the absence of external forces.