Law of Inertia states that: 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.

Galileo's experiments on the motion of object

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 experiments:

  • Experiments with single inclined plane
  1. Galileo observed that when an object moves down an inclined plane, its speed increases
  2. When the object is moved up the inclined plane, its speed decreases
  3. rom the above two observations, he argued that when the plane slopes neither upward nor downwards, there should be neither acceleration nor retardation.
  • Experiments on two inclined planes combined together: Galileo noted that in the case of an oscillating pendulum, the bob always reaches the same height on either side of the mean position. From this observation, he thought of an imaginary experiment. In this experiment, two inclined planes are facing each other:-
  1. 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.
  2. When the slope of the upward inclined plane is decreased, the object has to travel a longer distance to reach the maximum height. 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.
  3. From the above two observations, he argued that if the second plane is made horizontal, 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.