how interia came to realise by gallioe
Galileo studied motion of objects on an inclined plane. He found that
(i) objects moving down a smooth inclined plane accelerate, Fig. (a).
(ii) objects moving up a smooth inclined plane retard, Fig. (b).
(iii) objects moving on a frictionless horizontal plane move with a constant velocity, having neither acceleration nor retardation, Fig. (c).
In another experiment using a double inclined plane, Galileo observed that
A ball released from rest on one smooth inclined plane rolls down and climbs up the other smooth inclined plane. He found that
(i) in ideal situation, when there is no friction, the final height of the ball is the same as its initial height. In actual practice, when some friction is there, final height is somewhat less than the initial height. In no case, final height is ever greater than the initial height. When the slopes of the two planes are same, distance covered in rolling down one incline is the same as the distance covered in climbing up the other incline. This is shown in Fig. (a).
(ii) When the slope of second smooth inclined plane is decreased, and the experiment is repeated, the ball still reaches the same final height. But in doing so, it travels a larger distance as shown in Fig. (b).
(iii) When the slope of second smooth inclined plane is made zero (i.e., the second plane is made horizontal), the ball travels an infinite distance in the ideal situation (when there is no friction) i.e., motion of the ball never ceases, Fig. (c).
In actual practice, the ball comes to rest after moving some finite distance on the horizontal plane. This is because the opposing force of friction can never be totally eliminated.
From his experiments, Galileo concluded that the state of rest and the state of motion with constant velocity are equivalent. In both cases, no net force is acting on the body.
If net external force is zero, a body at rest continues to be at rest, and a body in uniform motion continues to move uniformly along the same straight line. This is Galileo's law of inertia.