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Work W = Fd =  The work done on a ____________

system. Work W = Fd =  The work done on a ____________ can going into changing its ________________energy PE, its ____________ energy KE, and/or its ____________ energy Q. D E T. potential. kinetic. internal. W. W. Fd. Fd. _____ due to _________ of entire object. KE.

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Work W = Fd =  The work done on a ____________

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  1. system Work W = Fd =  The work done on a ____________ can going into changing its ________________energy PE, its ____________ energy KE, and/or its ____________ energy Q. DET potential kinetic internal W W Fd Fd _____ due to _________ of entire object KE Q ___ within molecular and atomic ________ _____ stored up due to _________ motion PE bonds position "The System" + Q PE + KE Total energy of a system: ET = mechanical energy

  2. F box Gravitational ________________ PE - "position" energy an object has due to work done against ________________ gravity Raise a box at constant v: W done on system: F d w Dh mgDh F W = = = w=mg Dh Box has gained PE due to work done on it: DPE = mgDh

  3. mgDh DPE = PE PE direct direct m Dh PE PE constant direct v g

  4. Ex: Calculate the PE of a 3500-kg cockroach when it climbs to a height of 45 m above the ground. DPE = mgDh = (3500 kg)(9.81 m/s2)(45 m) = 1.5 x 106 kg m2/s2 Note: 1/ PE (like any energy) is a ________—it has no direction. 2/ The units: kg m2/s2 must equal 1______________. So the answer can also be written: DPE = or DPE = 3/ How much work did the cockroach do against gravity in order to increase its PE by this amount? scalar joule 1.5 x 106 J 1.5 MJ W = DPE = 1.5 MJ

  5. Sometimes this equation is written without the D's: PE = mgh The "h" or Dh" refers to the height above a zero _____________ level, especially during changes in height. reference Ex: a pendulum swings back and forth Using the floor as zero level: Dh = = Using the table top as zero level: Dh = = ceiling 1.6 m – 1 m 0.6 m – 0 m 0.6 m 0.6 m 0.6 m 1.6 m 0 m 1 m Table 1 m 0 m floor

  6. same Dh path Gravitational PE does not depend on __________ ________________ as the object is moved.  Both boxes below gain_________________________ of potential energy. traveled the exact same amount 5 kg 5 kg ground Dh is used to emphasize that it is only the ____________ ____________ in height that matters—not the path taken. vertical change

  7. Open your Review Book packet to pages 79-80. Do #26-38

  8. Kinetic F 1 kg __________ Energy KE - "motion" energy that an object has due to being ___________________________ accelerated to a speed v d DKE Work done W = = Fd Equation for kinetic energy: KE = (1/2)mv2 units: [KE] = (1/2) [ ] [ ]2 = ( )( )2 = = v m m/s kg kg m2/s2 a joule

  9. Ex. Determine the KE of Dobby, the 22-kg elf flying at a speed of 9.0 m/s at a height of 35 m above ground. KE = = = = (1/2) mv2 (1/2) (22 kg)(9.0 m/s)2 890 kgm2/s2 890 J Note: 1/ KE is a ____________ --it has no _______________ 2/ How much work was done on the elf in order to increase its KE to this amount? direction. scalar W = DKE = 890 J.

  10. KE = (1/2) mv2 KE Dh prop. to square direct KE KE v m Ex: A rabbit has 5.0 J of KE when running at a speed of 2.0 m/s. If the rabbit increases its speed to 4.0 m/s, what will the new KE be? double v  quadruple KE  4 x (5.0 J) = 20 J

  11. Ex. Two carts are released by a spring. Cart 1 has more mass than cart 2. 1 1 2 2 a/ Which cart gains more momentum? impulse same for both Dp = = = Ft t the same F because ____ and _____ are __________________ for both. b/ Which cart gains more kinetic energy? DKE = = = work done Fd different because the _____________ cart travels __________________ during the time the force is applied. This means more __________ is done to it, so its __________________________ . a greater d smaller KE is greater work

  12. Ex. What happens to kinetic energy in a perfectly elastic collision? wall vi vf = -vi (1/2)mvf2 KEbefore = (1/2)mvi2 KEafter = = KEbefore Ex. What happens to kinetic energy in a perfectly inelastic collision? wall vi 0 vf = 0 KEbefore = (1/2)mvi2 KEafter = kinetic heat energy The _________ energy was changed into _________________ .

  13. Ex: A ball is thrown straight up. What happens to its KE as it rises? As it falls? What is its KE when it reaches its maximum height? Compare its KE at the instant it is thrown up to its KE when it returns back to the same place that it was thrown from. Ex. A ball is thrown up at an angle. Is its KE at its highest point equal to 0? Explain. Ex. During uniform circular motion, why does the KE of an object remain constant? dec. inc. 0 same b/c v is same no still has horizontal v v F 900 W = Fdcosq = Fdcos900 = 0

  14. Open your Review Book packet to pages 84-85. Do #57-66

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