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You have already learned that you stub your toe harder trying to kick larger masses. Now imagine another unpleasant activity: catching a bowling ball. This gets harder to do as the ball is dropped from higher places. The difficulty depends both on the ball's mass and its velocity just before you apply the stopping force. This force can be applied in different ways. Any winner of an egg-throwing contest will tell you the way to stop an object with the least force is to spread the stopping process out over a maximum time.
This module will develop the above "folk physics" into a system of concepts and equations; and even a new law that is believed to be more fundamental than the laws from which it will be derived. This wonderful anomaly will not be further explored in this module, but it does indicate some of the philosophical richness and curiosity that continues to be part of this science. The concepts are "center of mass" and "linear momentum"; the law is called "conservation of linear momentum."