(13) Work and the Work-Kinetic Energy Theorem

(13) Work and the Work-Kinetic Energy Theorem - Lecture 13...

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Lecture 13 Work and the Work/Kinetic Energy Theorem
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Motivation to go beyond Newton’s laws Loop the loop R mg N N N The normal force has different direction and magnitude at every point on the track!! Writing and solving Newton’s laws can be a nasty experience… (see appendix of lecture 15)
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2 2 net , 1 1 2 2 x i f F x mv = - Playing around with Newton’s 2nd law. WORK (W ) done by force F over displacement Δ x Consider the motion of a bead of mass m on a straight wire pushed by a constant net force F x parallel to the wire, along a displacement Δ x: F net,x m Δ x x x F ma = Force produces acceleration: = - x i f a x v v v i v f Acceleration produces change in speed: x i f F x v v m = - Change in KINETIC ENERGY (K) of the bead
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This is an expression of the “effectiveness” of the force. net W K = ∆ Of how a force applied over a distance… …changes something in the system Work The “external agent” that changes the amount of kinetic energy (the state) in the system x W F x = Kinetic energy The “internal” quantity (state) of the system 2 1 2 K mv = 2 2 net , 1 1 2 2 x i f F x = - Work/Kinetic Energy Theorem
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m Δ x Initial kinetic energy K Final kinetic energy K > K Positive work W > 0 Energy source Work is energy being transferred F net,x
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Units for work and energy: SI: Not to be confused with: Calorie (or food calorie) 1 Cal = 1000 cal calorie 1 cal = 4.184 J Kilowatt-hour KWh Other common units: Joule 1 J = 1 N · m
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Energy Many types of energy: kinetic energy electric energy internal –thermal- energy elastic energy chemical energy Etc. Energy is transferred and transformed from one type to another and is never destroyed or created.
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What is work? The definition of work W =F Δ x
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(13) Work and the Work-Kinetic Energy Theorem - Lecture 13...

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