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Work W = the transfer of energy E

F. F. Work W = the transfer of energy E. Fd. W =. = D E T. on. increases. Work done ___ a system ____________ its total energy. Work done ___ a system ____________ its total energy. by. decreases. W done by system:. W done on system:. Box now has ______ energy. box.

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Work W = the transfer of energy E

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  1. F F Work W = the transfer of energy E Fd W = = DET on increases Work done ___ a system ____________ its total energy. Work done ___ a system ____________ its total energy by decreases W done by system: W done on system: Box now has ______ energy box Box now has ______ energy more less d d box pile

  2. F d

  3. Pile drivers:

  4. Ex. A 200-N box is pushed 8.0 m along a floor by a horizontal 75-N force. How much work was done? F = 75 N 200 N W = = = Fd (75 N)(8.0 m) 600 Nm joule • 1 newton·meter equals 1 ____________ : • 1 Nm = 1 ____ • So the answer can be written: W = _______ • Both Nm and J are ______________. • To express them in terms of fundamental units, • write: 1 J = 1 Nm J 600 J derived kgm/s2 m = 1 (________)(____) = 1 ______________ kgm2/s2

  5. Plot the force and distance from the previous example. F (N) 75 d (m) 8.0 What does the grey area represent? area = L W = ( )( ) = = area = LW = =  75 N 600 Nm 8.0 m 600 J the work done Fd W

  6. Note: 1/ In the equation: J = Ft, J represents the quantity_________. But in the answer: W = 600 J, J represents the unit____________. impulse joules • 2/ If you substitute F = 1 N and d = 1 m, you get: • W = Fd = ( )( ) = • 1 joule is the work done (or energy needed) in lifting 1_________________________to a height of 1 ___________. 1 J 1 m 1 N stick of butter meter energy 3/ Joules are metric __________ units. Americans use ____________ for energy units. Foods rated in calories in the US are rated in joules in the _______________________ . calories Rest of the World

  7. 1 m = 1 joule w ≈ 1 N F = 1 N w = 1 N

  8. No work.

  9. scalars 4/ Work W and energy E are __________. They do NOT have _______________, even though F and d do. direction 5/ What is the value of work W = Fd when d = 0? W = ____. This is b/c there is no _____ if d = 0. 0 DE 6/ The equation W = Fd is only valid when F and d are parallel: d d F or F

  10. F d When F and d are NOT parallel… …then the "real" equation for W must be used: W = where q is the angle between F and d Fdcosq q Fx= Fcosq F The equation W = Fd is ok to use here only if ____ is replaced by the ___________________ in the direction of d: W = Fd = _________ , which is the same as the one above. component of F Fcosqd

  11. Fdcosq The equation W = __________ explains why the work W that you do on a box is _________ when you carry it horizontally at _____________________ . zero constant velocity F you exert to hold W = Fdcosq = = = q = 900 Fdcos900 Fd(0) d w 0 Another way to see that W must equal 0: Given W = DET, doing work should change _________ .. But the energy E of box ________________________ as it is carried horizontally, so DET = ___ and so W = ____. energy does not change 0 0

  12. Ex. Which does more work, lifting a 30-N box a vertical distance of 2-meters at a constant speed of… …. 0.5 m/s or …1.0 m/s? 0 0 In both cases, constant v  a = ____  Fnet = ____. 30 N F exerted against gravity = _____ 30 N BOTH CASES: w = 30 N The W done against gravity: W = F d = ( ) ( ) = The distance 2 m will be covered in less _____ in case A, but W is ______________ for both cases. 2 m 30 N 60 Nm time the same

  13. What’s he doing? Working Time = power

  14. work is done the rate at which ______________________ the rate at which _________ is transferred Power P: energy P = W t no direction. scalar Power is a _____________—it has ___________________ If time t is constant: If work W is constant: P P inverse direct t W

  15. What did the motor say to the generator when he asked her out for a date? Don’t give me that old “power” line again.

  16. Ex: When Harry Potter lifts Draco Malfoy on his broomstick, 1400 J of work is done in 2.5 seconds. How much power did his broomstick develop? Unknown: P = ? Given: W = t = Equation: P = = = W/t 1400 J 1400 J/2.5 s 2.5 s 560 J/s watt • 1 joule/second equals 1 ___________ • 1 J/s = • So the answer can be written: P = • Both J/s and W are _________________ • To express them in terms of fundamental units, • write: 1 W = = = • = 1 W 560 W derived 1 Nm/s 1 J/s 1 (kgm/s2)(m)/s 1 kg m2/s3

  17. Oh, Quit it (ch)!

  18. Because work W = _____ and avg. speed vavg = ______ , the equation for power P can be written: P = W = = t Fd d/t Fd t F vavg Ex. How much time will it take a 1500-W (about 2 horsepower) motor to lift a 2200 N box a distance of 40. meters? Equation: Unknown: Given: P = Fd t t = ? P = 1500 W 1500 = (2200)(40) t F = 2200 N d = 40. m t = 59 s

  19. tide power:

  20. Notes: 1/ In the equation: P = W/t, W represents the quantity __________ But in the answer: P = 560 W, W represents the unit ____________ And in: w = 30 N w represents the quantity ___________ work watts weight • 2/ If you substitute F = 1 N, d = 1 m and t = 1 s, • you get: • P = Fd/t = ( )( )/ = = • 1 watt is the power developed in raising 1 _________ __________ to a height of 1 ________ in 1 ___________. 1 s 1 J/s 1 N 1 m 1 W stick of butter second meter 3/ To remember that 1 watt = 1 joule per second: joule second Watt? Don't worry, _______ get it in a _________!

  21. All that energy going up in smoke…

  22. And don’t forget..... Kimberly!

  23. Open up your Energy Packet to pages 77-8. Do #1-25 odd on a separate sheet of lined paper.

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