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

F d

Pile drivers:

Ex. A 200-N box is pushed 8.0 m along a floor by a horizontal 75-N force. How much work was done? 200 N F = 75 N W = = 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 = 1 (________)(____) = 1 ______________ joule J derived 600 J kgm/s 2 m kgm 2 /s 2 Fd (75 N)(8.0 m) 600 Nm

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

Note: 1/ In the equation: J = Ft, J represents the quantity _________. But in the answer: W = 600 J, J represents the unit ____________. 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 ___________. 3/ Joules are metric __________ units. Americans use ____________ for energy units. Foods rated in calories in the US are rated in joules in the _______________________. impulse joules 1 N 1 m 1 J stick of butter meter energy calories Rest of the World

1 m w ≈ 1 N F = 1 N w = 1 N = 1 joule

No work.

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

…then the "real" equation for W must be used: W = where  is the angle between F and d F d  F x = Fcos  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. When F and d are NOT parallel… Fdcos  component of F F Fcos  d

The equation W = __________ explains why the work W that you do on a box is _________ when you carry it horizontally at _____________________. F you exert to hold w d Another way to see that W must equal 0: Given W =  E T, doing work should change _________.. But the energy E of box ________________________ as it is carried horizontally, so  E T = ___ and so W = ____.   W = Fdcos  = Fdcos  zero constant velocity Fdcos90 0 Fd(0) 0 energy does not change 0 0

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? 30 N In both cases, constant v  a = ____  F net = ____. w = 30 N F exerted against gravity = _____ 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. BOTH CASES: N 2 m 60 Nm time the same

What’s he doing? Working Time = power

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

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.

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? Given : W = t = Unknown : P = ? Equation : P = = 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 (kgm/s 2 )(m)/s 1400 J 2.5 s W/t 1400 J/2.5 s 560 J/s watt 1 W 560 W derived 1 J/s 1 Nm/s 1 kg m 2 /s 3

Oh, Quit it (ch)!

Because work W = _____ and avg. speed v avg = ______, the equation for power P can be written: P = W = = t 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? P = Fd t Given: P = 1500 W F = 2200 N d = 40. m Unknown: Equation: t = ? 1500 = (2200)(40) t t = Fdd/t Fd t F v avg 59 s

tide power:

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 ___________ 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 ___________. 3/ To remember that 1 watt = 1 joule per second: Watt? Don't worry, _______ get it in a _________! joule second work watts weight 1 N1 m 1 s1 J/s 1 W stick of butter meter second

All that energy going up in smoke…

And don’t forget..... Kimberly!

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