18-kinetic energy and work 1

18-kinetic energy and work 1 - PHYS 211 3/2/09 Chapter 7...

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PHYS 211 3/2/09 Chapter 7 Kinetic Energy and Work Constant Forces Variable Force (Spring)
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Kinetic Energy Energy - property of a body (state of a system) Transfer energy with work Kinetic Energy - describes the state of motion K = 1 2 mv 2 Notes: scalar quantity never negative units: Joule (J) = kg m 2 /s 2
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Kinetic Energy Example An “alpha particle” with mass m=6.65E-27 kg is moving with a speed v=0.016c. What is the kinetic energy? Compare with kinetic energy of a slow moving train: m=1.2E5 kg, v=40 m/s K = 1 2 mv 2 = 1 2 (1.2 E 5 kg )(40 m / s ) 2 1 E 8 J K = 1 2 mv 2 = 1 2 (6.65 E 27 kg ) (0.016)(3.0 E 8 m / s ) [ ] 2 = 8.0 E 14 J
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Work The energy of a body changes if there is an exchange between a body and its environment Forces transfer energy -> work Positive work : apply a force to a body, it accelerates, v increases, K increases energy is transferred to the body Negative work : apply a force to a body, it decelerates, v decreases, K decreases energy is transferred from the body Δ K = K f K i = W
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Work-K.E. Theorem K f K i = W K f K i = F cos( θ ) d W =
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18-kinetic energy and work 1 - PHYS 211 3/2/09 Chapter 7...

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