Download Work, Power, and Machines

Work, Power, and Efficiency
Work
• is the transfer of energy that results from
applying a force over a distance
– if nothing moves, no work was done
• unit of measurement is the Joule (J)
– used to be a Newton-meter (N•m)
– (this is about equal to lifting quarter pounder
up 3 ft in a second)
• http://www.youtube.com/watch?v=IvcOY
OO0Fvw
Work formula
W=F·D
W = work
F = force
D = distance
Example problem
• How much work is done when an object with a
force of 60 Newtons is moved a distance of 7
meters?
given
F= 60 N
D= 7 m
formula
W=FD
set up problem
60N(7m)
answer w/ unit of measurement
420 Joules
• There are different “ways” to get
work done, but regardless, the work
must be done.
Work =
Force x
Work = Force x
Distance
Distance
What is the difference?
3m
3m
Power
• how “fast” work is being done
• unit of measurement is the watt (W)
– James Watt, a Scottish engineer, invented the steam engine.
– James Watt explained power as the number of horses his engine
could replace
• large quantities of power are measured in
kilowatts (kW)
– 1000 W = 1 kW
• Americans often use horsepower
– 1 horsepower = 746 watts
Power formula
P = W/T
P = power
W = work
T = time
Example problem
• A motor does 500 Joules of work in 5 seconds. How
much power does the motor have?
given
W= 500 J
T= 5 sec.
formula
set up problem
P=W/T 500 J / 5 sec.
answer w/ unit of measurement
100 watts
Efficiency
• the comparison of work put into a machine to the
work you get out of the machine.
– Work input (WI ) is the work that goes into a
machine
– Work output (WO ) is the work done by a
machine or the work you get out of the machine
• machines are always less than 100% efficient
because of friction
• however, they still help us by often reducing the
amount of force we “put in” by increasing the
distance we “put in”
Efficiency formula
Efficiency = WO (100)
WI
E = Efficiency
WO = Work Output
WI = Work Input
Example problem
• What is the efficiency of a machine that produces
150 Joules of work when 160 Joules of work is put
into it?
given
formula
set up problem
answer w/ unit of measurement
WO = 150J E = WO(100) 150J/160J(100)
WI = 160J
WI
94%