work w = the transfer of energy e w = work done ___ a system ____________ its total energy. work...
TRANSCRIPT
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Work W = the transfer of energy E
W =
Work done ___ a system ____________ its total energy.
Work done ___ a system ____________ its total energy
box
pilebox
d
Box nowhas ______energy
W done on system: W done by system:
d
F
Box nowhas ______ energy
Fd = ET
on increasesby decreases
moreless
F
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F
d
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Pile drivers:
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Ex. A 200-N box is pushed 8.0 m along a floor by a horizontal 75-N force. How much work was done?
200N
F = 75 NW =
=
=
•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/s2 m
kgm2/s2
Fd
(75 N)(8.0 m)
600 Nm
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Plot the force and distance from the previous example.
F (N)
d (m)
What does the grey area represent?
area = L W = ( )( ) = =
area = LW = =
75
8.0
8.0 m75 N 600 Nm 600 J
Fd W the work done
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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
energycalories
Rest of the World
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1 m
w ≈ 1 N
F =1 N
w =1 N
= 1 joule
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No work.
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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:
Fd
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.
scalarsdirection
0 E
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…then the "real" equation for W must be used:
W = where is the angle between F and d
F
d
Fx= 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 FF
Fcosd
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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
wd
Another way to see that W must equal 0:
Given W = ET, doing work should change _________ ..
But the energy E of box ________________________ as it
is carried horizontally, so ET = ___ and so W = ____ .
W = Fdcos
=
=
=
Fdcoszero
constant velocity
Fdcos900
Fd(0)
0
energy
does not change
0 0
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Ex. Which does more work, lifting a 30-N box a verticaldistance of 2-meters at a constant speed of…
…. 0.5 m/s or …1.0 m/s?
30N
In both cases, constant v a = ____ Fnet = ____.
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.
BOTHCASES:
0 0
30 N
30 N 2 m 60 Nm
time
the same
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What’s he doing?
Working Time
= power
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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
directP
t
inverse
If work W is constant:
Power P:work is done
energy
scalar no direction.
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What did the motor say to the generator when heasked her out for a date?
Don’t give me that old “power” line again.
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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/s2)(m)/s
1400 J
2.5 s
W/t
1400 J/2.5 s
560 J/swatt
1 W
560 W
derived
1 J/s 1 Nm/s
1 kg m2/s3
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Oh, Quit it (ch)!
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Because work W = _____ and avg. speed vavg = ______ ,
the equation for power P can be written:
P = W = = t
Ex. How much time will it take a 1500-W (about 2horsepower) 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 =
Fd d/t
Fd t
F vavg
59 s
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tide power:
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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 thequantity ___________
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 N 1 m 1 s 1 J/s 1 W
stick
of butter meter second
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All that energy going up in smoke…
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And don’tforget.....
Kimberly!
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Open up your Energy Packet to pages 77-8.
Do #1-25 odd on a separate sheet of lined paper.