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P H Y S I C S Newtons Laws Overview

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Review of Newtons Laws of Motion Objects in motion stay in motion* and objects at rest stay at rest if there is zero net force (balanced) ΣF = m·a (the forces will be unbalanced) Every force has an equal and opposite force * straight line/constant speed 1 st 2 nd 3 rd

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Inertia Depends on mass More mass more resistance Less mass less resistance

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Demo: NFL Hits

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Equilibrium Equilibrium: Net force is zero (ΣF = 0) ΣF x = 0 ΣF y = 0 FNFN FgFg F Air F Engine

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Equilibrium ΣF = 0 Newtons First Law applies An object in equilibrium can be: in motion (straight line/constant speed) at rest FfFf F engine FNFN FgFg F air

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Terminal Velocity Once the forces of air resistance and gravity become balanced equilibrium is reached No more acceleration

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Newtons Second Law

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Equations: F = ma a = F/m m = F / a F = net force m = Mass a= Acceleration

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Use one of the equations you just wrote down…

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Acceleration

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Weight vs Mass Weight Force F g F g = m ·g Mass: Amount of matter (does not change) Weight: Pull of gravity (changes)

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Weight Force (F g ) g = 9.8 m/s 2 g = 1.6 m/s 2 g = 26 m/s 2 m = 50 kg F g = 490 N ( 110 lb) m = 50 kg F g = 80 N ( 18 lb) m = 50 kg F g = 1300 N ( 292 lb)

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In-Class Problem #1 A 2000 kg car has a push force of 5000 N from its engine. If it experiences a friction force of 3000 N determine its (a) acceleration, (b) weight and (c) the normal force acting on it. a = 1 m/s 2 F g = 19,600 N F N = 19,600 N

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Review of Newtons Laws of Motion First Law a = 0 m/s 2 in motion*at rest stays in motion* stays at rest Second Law Accelerates depends inversely on mass depends on net force * Straight line/constant speed ΣF = 0ΣF 0

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Force that resists motion due to imperfections in surfaces FRICTION MOTION Friction

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Two Types 1.Static (rest): Keeps object from moving 2.Kinetic (moving): Slows moving object

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Friction Force Equation μ k (kinetic) μ s (static)

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Coefficient of Friction Table

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In-Class Problem #2 A 30 kg desk is at rest on the floor. It takes 200 N of force to start it in motion. Determine the static coefficient of friction between the desk and the floor. μ s = 0.68

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In-Class Problem #3 Once the desk in the previous problem is set in motion the 200 N force continues to be applied. Determine the acceleration of the desk if the coefficient of kinetic friction is a = 1.57 m/s 2

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