Power

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• Power
• Contents
• Definition of Power
• Whiteboards
• Jambalaya
• Whiteboards

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Power - The rate at which work is done

• Power Work/Time
• P W/?t
• P Power
• W - Work done
• ?t - Time to do work

A person does 145 J of work in 20.0 s. What is
power output? P W/?t (145 J)/(20.0 s) 7.25
J/s 7.25 Watts 7.25 W 1 horsepower 745.7
Watts, 1 kW 1000 Watts
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Power - The rate at which work is done

• power work/time (Fd)/t F(d/t) Fv
• F Force to move something
• v Velocity it is moving

P Fv My 30. HP van could go 25 m/s top speed.
What was the force resisting its motion? 1
horsepower 745.7 Watts, 1 kW 1000 Watts power
(30 HP)(745.7 Watt/HP) 22,371 W P Fv 22,371
W F(25 m/s), F 890 N
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Whiteboards Power 1 2 3 4 5
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Joe Mama does 613 J of work in 2.13 seconds.
What is his power output?
P W/?t (613 J)/(2.13 s) 288 W
W
288 W
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Ima Wonder can put out 127 W of power. What time
will it take her to do 671 J of work?
P W/?t, ?t W/P (671 J)/(127 W) 5.28 s
W
5.28 s
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What work does a 1.5 HP motor do in 1 minute?
P (1.5 HP)(745.7 W/HP) t 60 s P W/?t, W
Pt (1118.55 W)(60 s) 67113 J
W
67,000 J
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Bob N. Frappels slides a box with 43 N of force
at a constant speed of 5.3 m/s. What is his
power output?
P Fv (43 N)(5.3 m/s) 227.9 W 230 W
W
230 W
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Frieda People can put out 430 W of power. With
what speed can she push a car if it takes 152 N
to make it move at a constant velocity?
P Fv v P/F (430 W)/(152 N) 2.83 m/s
W
2.83 m/s
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Power all mixed up
Things we can know P - Power (in W) W - Work (in
J) F - Force (in N) d - distance (in m) t - time
(in s) ? - coefficient of friction m - mass (in
kg) v - Velocity (in m/s) a - acceleration (in
m/s/s)
Formulas F mg (Lifting) F ?mg
(Dragging) F ma (Accelerating) P W/t
(Power) P Fv (Power) W Fd (Work) v d/t
(Velocity) a ?v/t (Acceleration)
Jambalaya!
Its a
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P - Power (in W) W - Work (in J) F - Force (in
N) d - distance (in m) t - time (in s) ? -
coefficient of friction m - mass (in kg)
Formulas F mg (Lifting) F ?mg
(Dragging) P W/t (Power) W Fd (Work)
Sample 1 - A 1.2 HP winch can lift a 2350 kg
LandRover up 14.5 m into a tree in what time?
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P - Power (in W) W - Work (in J) F - Force (in
N) d - distance (in m) t - time (in s) ? -
coefficient of friction m - mass (in kg)
Formulas F mg (Lifting) F ?mg
(Dragging) P W/t (Power) W Fd (Work)
Sample 2 - Gumi Baere drags a 45.1 kg box that
has a coefficient of friction between it and the
floor of .34 a distance of 16 m in 11.7 seconds.
What is her power output?
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Whiteboards Power Jambalaya 1 2 3 4
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What must be the power rating of a motor if it is
to lift a 560 kg elevator up 3.2 m in 1.5 seconds?
F mg (560 kg)(9.8 N/kg) 5488 N W Fd
(5488 N)(3.2 m) 17561.6 J P W/t (17561.6
J)/(1.5 s) 11707.7 W
W
11700 W
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A .75 HP winch can lift a car 5.2 m in 37
seconds. What must be the mass of the car? 1 HP
745.7 W
P (.75 HP)(745.7 W/HP) 559.275 W P W/t, W
Pt (559.275 W)(37 s) 20693.175 J W Fd, F
W/d (20693.175 J)/(5.2 m) 3979.5 N F mg, m
F/g (3979.5 N)/(9.8 N/kg) 406 kg
W
406 kg
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Red Elk leads a dog team that can put out 2.5 kW
of power. They skid a 312 kg sled a distance of
340 m in 93 seconds. What must be the
coefficient of friction?
P 2500 W P W/t, W Pt (2500 W)(93 s)
232500 J W Fd, F W/d (232500 J)/(340 m)
683.8 N F ?mg,? F/(mg) (683.8 N)/((312
kg)(9.8 N/kg)) .22
W
.22
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A 150 HP tractor can drag a 350 kg load how far
in a minute if the coefficient of friction
between the load and the ground is .78. 1 HP
745.7 W
P (150 HP)(745.7 W/HP) 111855 W F ?mg
(.78)(350 kg)(9.8 N/kg) 2675.4 N P W/t, W
Pt (111855 W)(60 s) 6711300 J W Fd, d W/F
(6711300 J)/(2675.4 N) 2508 m
W
2500 m