Download 02-01Power

Power
Contents:
•Definition of Power
•Whiteboards
•Jambalaya
•Whiteboards
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
TOC
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
TOC
Whiteboards:
Power
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TOC
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
288 W
W
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
5.28 s
W
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
67,000 J
W
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
230 W
W
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
2.83 m/s
W
Power all mixed up
Formulas:
F = mg (Lifting)
F = mg (Dragging)
P = W/t (Power)
W = Fd (Work)
It’s a
Things we can know
P - Power (in W)
W - Work (in J)
F - Force (in N)
d - distance (in m)
t - time (in s)
m - mass (in kg)
 - coefficient of friction
Jambalaya!
Formulas:
F = mg (Lifting)
F = mg (Dragging)
P = W/t (Power)
W = Fd (Work)
P - Power (in W)
W - Work (in J)
F - Force (in N)
d - distance (in m)
t - time (in s)
m - mass (in kg)
 - coefficient of friction
Sample 1 - A 840 W winch can lift a 2350 kg Land
Rover up 8.2 m into a tree in what time?
Formulas:
F = mg (Lifting)
F = mg (Dragging)
P = W/t (Power)
W = Fd (Work)
P - Power (in W)
W - Work (in J)
F - Force (in N)
d - distance (in m)
t - time (in s)
m - mass (in kg)
 - coefficient of friction
Sample 2 - Gumi Baere drags a 45.1 kg box that
has a coefficient of friction between it and the
floor of 0.34 a distance of 16 m in 11.7 seconds.
What is her power output?
Whiteboards:
Power
Jambalaya
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TOC
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
11700 W
W
A 560. W 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 = W/t, W = Pt = (560 W)(37 s) = 20,720 J
W = Fd, F = W/d = (20,720 J)/(5.2 m) = 3984.6 N
F = mg, m = F/g = (3984.6 N)/(9.8 N/kg) = 406.6 kg
407 kg
W
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
= .22
W
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 0.78.
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
2500 m
W