135 Notice that here “persevere in their motions” must mean in steady speed straight line motions, because he is adding the gravitational acceleration on to this. This is sometimes called “The Law of Inertia”: in the absence of an external force, a body in motion will continue to move at constant speed and direction, that is, at constant velocity. So any acceleration, or change in speed (or direction of motion), of a body signals that it is being acted on by some force . 20.10 Newton’s Second Law: Acceleration of a Body is Proportional to Force Newton’s next assertion, based on much experiment and observation, is that, for a given body , the acceleration produced is proportional to the strength of the external force, so doubling the external force will cause the body to pick up speed twice as fast. Law 2 The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed. 20.11
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This note was uploaded on 01/08/2012 for the course PHY 322 taught by Professor Daser during the Spring '09 term at SUNY Stony Brook.