9-15- Newton'sLawsII - Newtons second law of motion When a...

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Newton’s second law of motion When a net force acts on an object, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass (m) of the object.” m net F a r r = a m F net r r = Unit of force: 1 Newton = 1 Kg m/s 2 y y x x ma F ma F = Σ = Σ Newton’s 2 nd law in components: F r Σ a m r = Newton’s third law of motion “Whenever a body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body.” Characteristics of action-reaction On different objects Same magnitude, opposite direction Same type on fridge by hand on hand by fridge on shuttle by earth on earth by shuttle
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ON the object, not forces exerted by the object on the surroundings. Action – reaction Earth Weight (on book by earth) force on book by hand force on hand by book on earth by book Action – reaction Never include both action and reaction in the same free-body diagram. Free-Body Diagrams revisited Problem A 54-kg parent and a 18-kg child meet at the center of the ice rink and push against each other. (a) Is the force experienced by the child more than, less than, or the same as the force experienced by the parent? (b) Is the acceleration of the child more than, less than, or the same as the acceleration of the parent? (c) If the acceleration of the child is 2.6 m/s 2 , what is the acceleration of the parent? (c) On Child: magnitude of force F = m a = (18kg)(2.6m/s 2 ) = 46.8N On Adult: magnitude of force =46.8 N 18 kg 54 kg Third law: action – reaction same magnitude Second law: under same magnitude of force, F r F - r a r /3 a - r Child—smaller mass (inertia)--- larger acceleration
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9-15- Newton'sLawsII - Newtons second law of motion When a...

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