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lecture chapter 7

lecture chapter 7 - Chapter 7 Work and Kinetic Energy...

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1 Chapter 7. Work and Kinetic Energy Force exerted through a distance performs mechanical work. Which one costs energy? 1. Constant force in direction of motion. W=F d SI unit : Joule 1 J = N.m Experiment on the front desk , Pull, rest, lift My hand : F= 15 N, distance = 2 m , W=30 J 2. Constant force at an angle θ to motion W= F d cos θ Experiment on the front desk, Pulling at θ = 20 , F=15N, d=2 m W= Fd cos θ =15*2* cos 20 = 28 J Question: how to through a baseball to give it large speed? Answer: Apply force across a large distance! (try it)

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2 What if θ =90 ? cos θ = 0 W = 0 Normal force What if θ =180 ? cos θ = -1 W = - F d kinetic Friction! 3. Negative Work: when force and motion are in opposite directions . 90 < θ 180 Examples: to catch a baseball, to stop a car 4, Total work: Wtotal = W1 + W2 +W3+ …. . the sum of the work done by each forces. Constant force at an angle θ to motion W= |F| |d| cos θθ is angle between F & motion The desk example again: If μ k = 0.28, μ s = 0.30, mg = 49 N, if moved d=2m My hand pull F= 15 N Pulling work: W Pull = F . d = 15*2 =30 J > 0 Table surface f k = μ k . N = μ k mg = 0.28 x 49 = 13.7N Friction work: W fk = f k
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lecture chapter 7 - Chapter 7 Work and Kinetic Energy...

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