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Today: Dynamics I Mass, Inertia, Weight Balanced Forces Newton’s Laws Free-body diagrams
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Aristotle v. Newton 384 – 322BC 1643 – 1747AD
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Aristotle’s Laws of Motion Nothing moves unless you push it. [it is moved by a mover] »Some motion is natural for the sublunar elements, rectilinear motion to or away from the earth's center for the supralunar quintessence, circular motion »All other motion is violent, and requires a mover [Anselm's nth proof of the existence of God] Because motion exists, there must be a self-moved mover, i.e. a Prime Mover [later i.d. God] There is natural, violent, and local motion; rectilinear and circular motion Speed is proportional to motive force, and inversely proportional to resistance. »v = k (F / R) There cannot be a vacuum [therefore Natura is a plenum] The most Natural state: Rest [if terrestrial]
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Newton’s First Law
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Objects in motion tend to stay in motion, and objects at rest tend to stay at rest unless an outside force acts upon them.
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Inertia Kitchen roll Coin and cup A hammerhead Anvil and hammer Airplanes Weight?
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Zero Net Force (or, a delicious melon)
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Newton’s Second Law The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
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Newton’s Second Law in symbols F = m a (unit: Newton / kg m s -2 ) W = ?
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Try: A car has a mass of 1000 kg. At one point in its motion, when the combined forces of air resistance and friction (acting backwards) are 500 N, its acceleration is 1.6 m s -2. What forward driving force does it need? Car (obviously) Air resistance + Friction Driving force Direction of motion
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Newton’s Third Law When body A exert a force on body B, body B exerts a force on body A that is equal in magnitude and opposite in direction
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Delicious-melon-on-table
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Delicious-melon-on-table- on-planet
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Think: What action-reaction pairs can you think of happening right now?
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Free-Body Diagram Magnet-string-paperclip
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Free-body diagrams RI boy’s head sleeping on a table RI boy suspended motionless from a bar hanging from a ceiling by two ropes RI boy free-falling from a nest in a tree. Neglect air resistance. RI boy gliding from a tree to the ground at constant velocity. Consider air resistance.
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Free-body diagrams A rightward force is applied to a RI boy in order to move it across a desk with a rightward acceleration. Same as above, but constant velocity. A RI boy rests a backpack on his shoulder. The pack is suspended motionless by one strap from one shoulder. A skydiving RI boy is descending with constant velocity. Consider air resistance.
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Free-body diagrams A force is applied to the right to drag a RI boy across loosely- packed snow with rightward acceleration. A RI boy is flying upwards after having been punted by a teacher. Neglect air resistance. A car is coasting to the right and slowing down (no RI boys involved).
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Conclusion Mass, Inertia, Weight Balanced Forces Newton’s Laws Free-body diagrams
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