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Published byArmando Stockton Modified over 2 years ago

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Aim: What are Newtons 1 st and 3 rd Laws of Motion? Do Now: What is the weight of the box? m = 20 kg F g = mg F g = (20 kg)(9.8 m/s 2 ) F g = 196 N

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So why isnt the box accelerating downward? No acceleration means no unbalanced forces In other words: Forces are balanced F Net = 0 The table is pushing up with an equal force This is called the Normal Force (F N ) F g = 196 N F N = 196 N

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Normal Force Represented by F N Represented by F N Supporting force from the surface the object is on Supporting force from the surface the object is on Perpendicular and away from the surface Perpendicular and away from the surface If object is at rest in that direction, F N is equal to the objects weight If object is at rest in that direction, F N is equal to the objects weight FgFg FNFN

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Newtons 1 st Law of Motion Law of Inertia If all the forces acting on an object are balanced (F Net = 0): a. an object at rest remains at rest Penny in beaker Demo

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b. an object in motion will remain in motion with a constant velocity

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Inertia – the ability of an object to resist a change in motion Objects at rest want to remain that way Objects in motion want to remain that way Inertia is dependent only on mass (the more mass, the more inertia)

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A boy is pulling a 30 kg sled with a force of 100 N at a constant velocity Draw a free – body diagram to illustrate this F g = 294 N F N = 294 N F = 100 N Are we missing any forces? F F = 100 N Force of friction (F f ) – opposes the direction of motion Forces must be balanced

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Object is at rest. What are the values of Forces A and B? 5 N 20 N Force A = Force B = 20 N 5 N

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Newtons 3 rd Law – Law of Action – Reaction For every action there exists an equal but opposite reaction Demo – Balloon What is the action? What is the reaction?

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Action – Jumper pushes on floor Reaction – floor pushes on jumper

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Action – Foot pushes backward on floor Reaction – Floor pushes forward on foot

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Action – Boat pushes person forward Reaction – Person pushes boat backwards

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Action – Rocket pushes the air down Reaction – Air pushes the rocket up

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