10_1425_web_Lec_06_Newton

10_1425_web_Lec_06_Newton - Newtons Laws Physics 1425...

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Newton’s Laws Physics 1425 lecture 6 Michael Fowler, UVa.
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Newton Extended Galileo’s Picture of Motion to Include Forces Galileo said: Natural horizontal motion is at constant velocity unless a force acts: a push from behind will cause acceleration, friction will cause negative acceleration (that is, deceleration). Natural vertical motion is constant downward acceleration…
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Newton Said They’re the Same Thing! The “natural vertical motion” is at constant acceleration because there’s a constant force acting – the force of gravity ! Without that force, vertical motion would be at constant velocity. Look again at the path of a projectile : without gravity, it would be a straight line.
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Vector Picture of Projectile Motion Position at 1 second intervals (notice it falls below straight line : the g =0 trajectory ). Velocities and Speeds at 1 second intervals. 2 1 0 2 r v t gt =  0 r vt =
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Newton’s First Law of Motion Newton’s First Law is that an object continues to move at constant velocity unless acted on by external forces. Unlike Galileo’s horizontal motion law, this applies for motion in any direction . (This was hard to accept, because forces were considered to arise only from contact , a push or pull, and this “force of gravity” seemed magical.)
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Relating Change in Velocity to Force This can only be done experimentally : Newton did many experiments.
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10_1425_web_Lec_06_Newton - Newtons Laws Physics 1425...

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