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Lecture # 3 Cassandra Paul Physics Summer Session II 2008

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Today Work Mechanical Energy Systems Force Graphing

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Heat and Work Let’s Quickly review heat so we can differentiate it from work… 0 0 C Ice-cube Water An ice-cube sits in a bath of water. Water and ice can exchange heat with each other but not with the environment. What is the direction of heat transfer? A) From ice-cube to water B) From water to ice- cube C) Impossible to tell D) Neither of above

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No Heat Transfer! Temperature (K) Energy added (J) solid liquid gas Temperature (K) Energy added (J) solid liquid gas IceWater

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Heat Transfer Heat Transfer can only happen if there is a Temperature difference. Low tempHigh temp Heat Heat is a transfer of energy (a process) that takes place from a hot object to a cold one because the objects are at different temperatures. Energy leaves hot objects in the form of heat Energy enters cold objects in the form of heat

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Work Work is done whenever a force is exerted. KESpeed Baseball Work The pitcher’s hand “pushed” the baseball. The pitcher’s hand exerted force on the baseball. As a result, the baseball started moving (its KE increased). Work Work is a transfer of energy (process) that takes place from a physical system to another physical system due to an interaction that involves “Force”.

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What are some examples of when work is done? Pushing Lifting Even Falling?

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Work changes Mechanical Energies Energy specifically due to motion of ‘everyday things.’ Kinetic Energy (Translational) Gravitational Potential Energy Spring Potential Energy Sweet! New bubbles to put in my energy interaction diagrams!!!! PE gravity PE spring KE trans Speed Height Displacement from EquilibriumX

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Work is done when there is Force To be more precise, we need the concept of “Force” : “Push” or “Pull” An overall push (or pull!) in the direction the object is travelling has the effect of speeding it up. Block is already moving, you push in same direction: direction of travel direction of Force KESpeed Work Consider a block being pushed by you on a level surface with no friction: W=ΔKE=F d

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Block is already moving, you push in same direction: direction of travel direction of Force KESpeed Work Consider a block being pushed by you on a level surface with no friction: W=ΔKE=F d What does this d mean? A)The distance the block travels B)The distance the force is exerted over

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Force Properties of forces Force is a vector quantity i.e. Forces have both magnitude and direction Force is the agent of interaction of TWO objects e.g. The pitcher’s hand and the baseball The two forces involved in an interaction are opposite and equal (Newton’s Third Law) F hand on the baseball = - F baseball on the hand More on this in 7B!

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Force Properties of forces Force is a vector quantity i.e. Forces have both magnitude and direction Force is the agent of interaction of TWO objects e.g. The pitcher’s hand and the baseball The two forces involved in an interaction are opposite and equal (Newton’s Third Law) Contact force vs non contact force F gravitational

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Gravity is a force, therefore you can model a ball falling as an open system! Ignoring Friction, find the amount of work done by gravity on the ball as it falls from a height of 10 meters to the floor. Ignoring Friction, find the final speed of a ball just before it hits the floor after it falls from a height of 10 meters to the floor. KE trans Speed Work + + ΔKE = W KE trans Speed PE gravity Height ΔKE +ΔPE= 0 7A way convention… …but nothing wrong with this way too!

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Work can enter or leave a system Example: A book is initially at rest, you slide the book across the table to your friend. It stops right in front of your friend. KE trans Speed KE trans Speed System: Book Initial: Book is at rest (right before push) Final: Book is at highest speed (right after push) System: Book Initial: Book is at highest speed (right after push) Final: Book is at rest (book has stopped) Work ΔKE = W Work done by hand Work done by friction + +--

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Work Example: A pitcher throws a 0.3kg baseball 44m/s (100mph) how much energy is transferred from the pitcher’s hand in the form of work? KESpeed System: Baseball Initial: Ball at rest in pitcher’s hand Final: Ball just leaves the pitcher’s hand Work ∆KE = Work KE final - KE initial =1/2(m)(v f 2 ) – 0 = W (0.5)(0.3kg)(44m/s) 2 = 290.4 J

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Intro to Graphing PE and KE Tuesday you will be doing some graphing, let’s practice.

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Diving: Potential Energy 0m or 3m 2m or 5m -2m or 1m -3m or 0m At highest point, Tricia Woo is 2 meters above the board and 5 meters above the water, how should we calculate her PE? Where should we measure the height from? From board From floor

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KE trans Speed PE gravity Height ΔKE +ΔPE= 0 System: Diver Initial: Highest point Final: Just before hitting water We want to make sure to calculate the correct final velocity for the diver, where should we set the height equal to zero? A) 0m at top B) 0m at board C) 0m at water D) It doesn’t matter E) Need more information

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How can it not Matter!? KE trans Speed PE gravity Height ΔKE +ΔPE= 0 System: Diver Initial: Highest point Final: Just before hitting water ½ m(v f 2 -v i 2 ) + mg(h f -h i )= 0 (0.5)(50kg)(v f 2 -0) + (50kg)(10m/s 2 )(h f -h i )= 0 0m or 3m 2m or 5 m -2m or 1m -3m or 0m From board From floor (0 - 5) (-3 - 2) Δh is the same! Δh=-5 so v f = 10m/s +-

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Instantaneous PE and KE ΔKE +ΔPE= 0 (KE f – KE i ) + (PE f - PE i ) = 0 KE f + PE f - KE i - PE i = 0 KE f + PE f = KE i + PE i = Etot KE anytime + PE anytime = Etot The sum total of all of the energies at one point in time is equal to the total energy of the system. In a closed system that value is constant throughout the process.

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Equations to memorize and more importantly know how to use this week ½ mΔ(v 2 )= ½ m (v f 2 -v i 2 ) = ΔKEtrans mgΔh = ΔPEgrav ½ k(Δx f 2 -Δx i 2 ) = ΔPEspring

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DL sections Swapno: 11:00AM EversonSection 1 Amandeep: 11:00AM Roesller Section 2 Yi: 1:40PM Everson Section 3 Chun-Yen: 1:40PM Roesller Section 4

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Physics 7A -- Lecture 3 Winter 2008

Physics 7A -- Lecture 3 Winter 2008

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