power
DESCRIPTION
Objective : To define and apply the concepts of power and mechanical efficiency. Power. The rate at which work is done. The power is the work done divided by the time it takes to do the work. Units: J/s = Watt 1 horsepower =746 Watts 1000 Watts = 1 kW. - PowerPoint PPT PresentationTRANSCRIPT
Power
The rate at which work is done.
Objective: To define and apply the concepts of power and mechanical efficiency.
The power is the work done divided by the time it takes to do the work.
t
WP
Units:
J/s = Watt
1 horsepower =746 Watts
1000 Watts = 1 kW
Kilowatt-hours (kwh) measure energy.
Jss
Jkwh 6106.3360010001
SRP charges about 10 cents for 1kwh. How much would it cost to leave a 60W light bulb on for a full day?
kWW 060.060
)24)(060.0( hourskW
kWh44.1
centskwh
centskwh 4.14
1044.1
Leaving a 60W light bulb on for 24 hours will cost about 15 cents.
This equation can be written in another form:
t
WP
t
xFP
cos
cosxFxFW
cosFvP
Which has more power?
A 60kg man takes 5 seconds to run up a set of stairs at a constant velocity. What was the man’s power?
t
U
t
WP g
t
mgh
15m
Mass = 60kg
Work is transferred to gravitational potential Energy
5
)15)(10)(60(
)(1800 Wattss
J
A man pushes a 12kg crate horizontally with a constant velocity of 2m/s. The coefficient of friction between the crate and the floor is 0.25. What power does the man have as he pushes the crate? cosFvP
)1)(2)((FP
-120N
120N
Ff
Nf
f
Ff
30
)120)(25(.
)1)(2)(30(P
WP 60
A crane hoist lifts a load of 1000kg a vertical distance of 25m in 9.0s at a constant velocity. What is the useful power output of the crane?
t
mgh
t
U
t
WP g
WP 4108.29
)25)(10)(1000(
If the hoist motor of the previous crane is rated at 70hp, what percentage of this power goes to useful work?
Whp
whp 4102.5
1
74670
W
W4
4
102.5
108.2%
%54
Efficiency – A measure of what you get out for what you put in.
in
out
E
W
inputenergy
outputwork
The Efficiency of a typical car is about 25%!
The motor of an electric drill with an efficiency of 80% has a power of input of 600 W. How much useful work is done by the drill in a time of 30 seconds?
Jss
JW 18000)30(600600
in
out
E
W
inputenergy
outputwork
1800080.0 outW
JWout 14400