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Transcript
Lecture # 3 Cassandra Paul Physics Summer Session II 2008 Today • • • • Work Mechanical Energy Systems Force Graphing Heat and Work Let’s Quickly review heat so we can differentiate it from work… Ice-cube 00 C Water 00 C 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 icecube C) Impossible to tell D) Neither of above No Heat Transfer! Ice Water Temperature (K) Temperature (K) gas gas liquid liquid solid solid Energy added (J) Energy added (J) Heat Transfer Heat Transfer can only happen if there is a Temperature difference. 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. Low temp High temp Energy leaves hot objects in the form of heat Energy enters cold objects in the form of heat Work Work is done whenever a force is exerted. 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”. Baseball KE Work Speed 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). What are some examples of when work is done? Pushing Lifting Even Falling? 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!!!! KEtrans Speed PEgravity Height PEspring Displacement X from Equilibrium 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. Consider a block being pushed by you on a level surface with no friction: Block is already moving, you push in same direction: direction of Force direction of travel KE Work Speed W=ΔKE=Fd Consider a block being pushed by you on a level surface with no friction: Block is already moving, you push in same direction: direction of Force direction of travel KE Work Speed W=ΔKE=Fd What does this d mean? A)The distance the block travels B)The distance the force is exerted over 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) Fhand on the baseball = - Fbaseball on the hand More on this in 7B! 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 Fgravitational Gravity is a force, therefore you can model a ball falling as an open system! 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. 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. Work KEtrans Speed PEgravity Height KEtrans Speed + ΔKE +ΔPE= 0 7A way convention… + ΔKE = W …but nothing wrong with this way too! 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. 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 Work done by hand KEtrans Speed + + ΔKE = W Work Work done by friction KEtrans Speed - - ΔKE = W 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? System: Baseball Initial: Ball at rest in pitcher’s hand Final: Ball just leaves the pitcher’s hand KE Work Speed ∆KE = Work KEfinal - KEinitial =1/2(m)(vf2) – 0 = W (0.5)(0.3kg)(44m/s)2 = 290.4 J Intro to Graphing PE and KE Tuesday you will be doing some graphing, let’s practice. Diving: Potential Energy From board 2m From floor or 5m 0m or 3m -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? 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 KEtrans Speed PEgravity Height B) 0m at board ΔKE +ΔPE= 0 C) 0m at water D) It doesn’t matter E) Need more information How can it not Matter!? System: Diver Initial: Highest point Final: Just before hitting water KEtrans Speed + From board 2m PEgravity From floor or 5m 0m or 3m -2m or 1m Height - ΔKE +ΔPE= 0 -3m or ½ m(vf2-vi2) + mg(hf-hi)= 0 (0 - 5) (0.5)(50kg)(vf2-0) + (50kg)(10m/s2)(hf-hi)= 0 Δh is the same! Δh=-5 so vf = 10m/s(-3 - 2) 0m Instantaneous PE and KE ΔKE +ΔPE= 0 (KEf – KEi) + (PEf- PEi) = 0 KEf + PEf - KEi - PEi = 0 KEf + PEf = KEi + PEi = 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. KEanytime + PEanytime = Etot Equations to memorize and more importantly know how to use this week mgΔh = ΔPEgrav ½ mΔ(v2)= ½ m (vf2-vi2) = ΔKEtrans ½ k(Δxf2-Δxi2) = ΔPEspring DL sections • • • • Swapno: 11:00AM Everson Section 1 Amandeep: 11:00AM Roesller Section 2 Yi: 1:40PM Everson Section 3 Chun-Yen: 1:40PM Roesller Section 4