By observing the motion of objects on inclined planes, Galileo deduced that objects move with constant speed when no force acts on them.
In his first experiment, Galileo studied the motion of marbles on an inclined plane. He observed that when a marble rolls down an inclined plane, its velocity increases, as shown in Fig. 9.6 (a). Here the marble falls under the unbalanced force of gravity. The velocity of the marble decreases when it rolls up the inclined plane (against the force of gravity), as shown in Fig. 9.6 (b). From these observations, Galileo argued that the velocity of a marble rolling on flat horizontal surface should remain constant.
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