Power
1
Work has to do with a force causing a displacement.
Work has nothing to do with the amount of time that this force
acts to cause the displacement.
Power is the rate at which work is done. How fast is the work
being done.
It can be found using the following equation.
P ow er 
W ork
T im e
2
The metric unit of power is the Watt ( W ).
As is implied by the equation for power, a unit of power is
equivalent to a unit of work divided by a unit of time, thus, a
Watt is equivalent to a Joule/second
For historical reasons, the horsepower is occasionally used to
describe the power delivered by a machine. One horsepower is
equivalent to approximately 750 Watts.
3
Example:
In preparation for the next power outage Mr. Harper is
trying to set up a generator. If the generator is capable of
putting out 9.5 kW of power, what size of engine (in hp)
will he need in order to run the generator?
9.5 kW 
1000W
1kW

1hp
750W
 12.6666  13 hp
Example:
A crane is capable of doing 1.5 x 105 J of work in 10 s.
What is the power of the crane in watts?
P 
W
t

1.5  10
5
 1.5  10 W
4
10
5
Example: A cyclist and her mountain bike have a combined
mass of 60 kg. She’s able to cycle up a hill that changes her
altitude by 400 m in 1min.
a) How much work did she do against gravity in climbing
the hill?
W  fd  w h  m gh   60   9.8   400   240 kJ
b) How much power is she able to generate?
P
W
t

2.4  10
5
 3.9  10 W
3
60
6
Example: Two physics students, Jacob and Ryan are in the weight
lifting room. It takes Jacob 3 sec to lift the 100 kg barbell over his
head a distance of 0 .75 m. It takes Ryan 2 sec to lift the same
barbell over his head a distance of 0.55 m.
a) Which student does the most work?
b) Which student delivers the most power?
7
Do
Practice Problems Page 266 (pdf 37) #’s 41 - 43
Horsepower Lab (pdf 37) Page 267
Read pg 268 (pdf 37) and make up your own
notes on efficiency
8
Efficiency
Efficiency is the ratio of useful energy or work output to the
total energy or work input.
E fficiency 
Eo
 100%
Ei
E fficiency 
Wo
 100%
Wi
9
Example: A model rocket engine contained explosives stores
3.50x103 J of chemical potential energy. When launched the
stored chemical energy is transformed into gravitational
potential energy. Calculate how efficiently the rocket
transforms the stored chemical energy into gravitational
potential energy if the 0.50 kg rocket is propelled to a height
of 100 m.
14%
10
Do
Practice Problems Page 270 (pdf 37) #’s 44 - 50
** EXTRA QUESTIONS **
(only if students want to)
End of Chapter Review Pg 274 (pdf 38)
#’s 1, 2, 4, 5, 6, 15, 16, 17,18, 20, 21, 23, 24, 25, 26, 27, 28
11