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 obser