# physics - Physic 231 Lecture 7 Main points of todays...

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Physic 231 Lecture 7 Main points of today’s lecture: Work: () x F s F W x Δ = Δ = θ cos Kinetic energy: 2 1 mv KE = θ s Δ F Work-energy theorem: 2 0 KE KE W f total = Potential energy: Conservation of energy ( ) 0 0 y y mg PE PE PE f f = = Δ 0 0 PE KE PE KE E f f + = + =

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Example kinetic friction with ramp The block shown below starts sliding down the ramp. Assuming the efficient of kinetic friction = 0 3 how long does it take for the coefficient of kinetic friction μ k 0.3, how long does it take for the block to travel 2m to the bottom of the ramp ? 1. Draw the forces. hoose an appropriate N k f 2. Choose an appropriate coordinate system. 3. Calculate the components. . se Newton’s2 nd get W 40 0 40 0 W k W,f , N are magnitude of forces 4. Use Newton s 2 to get t () i,x x k i x: F ma f Wsin θ == i y W x kinetic friction ( ) kx N mgsi nm a μθ i,y i y: F 0 N Wcos m g c o s μ k = 0.3 v o =0 ( ) Nm oo mg cos 40 mgsin 40 ma μ // / 2 x a4 . 0 5 m / s θ =40 o 2 x 2 x 12 x 4 m xa tt 1 s 2 a 4.05m / s Δ− Δ=
Reading Quiz . a system is olated he total energy of the system 1. If a system is isolated , the total energy of the system A. increases constantly. ecreases constantly B. decreases constantly. C. is constant. D. depends on work into the system. epends on work out of the system E. depends on work out of the system. Slide 10-6

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Reading Quiz . you raise an object to a greater height, you are increasing 3. If you raise an object to a greater height, you are increasing A. kinetic energy. eat B. heat. C. potential energy. D. chemical energy. ermal energy E. thermal energy. Slide 10-10
Work Physicists have a precise definition of work done by a force on an object. It is computed from the displacement of the object while being acted on by the force. () x F s F W Δ = Δ = θ cos θ F s Δ Note: work is only done if there is a displacement in the direction of the force. = 0 o W = F if θ 0 , W F Δ s –i f θ = 90 o , W = 0. f θ = 180 o , W = -F Δ s •T he units for work are N m = J (Joules). These are the units we will use in this chapter. Other units are cal (calorie). 1 cal = 4.186J. Calorie (Food calories) is another unit. 1 Calorie = 1000 cal. Work is not the same thing as effort.

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What work is being done by me? Indicate with your response unit whether: – a) positive work is being done – b) no work is being done – c) negative work is being done What work is being done by gravity?
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## This note was uploaded on 05/03/2011 for the course PHYS 231 taught by Professor Nagy during the Fall '11 term at Michigan State University.

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physics - Physic 231 Lecture 7 Main points of todays...

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