81m/s2
.
.
Example 1
A building hoist lifts ten 50kg bags cement the top 30m high building.
Work done Force Distance moved (J)
but force Mass c
cceleration (N)
Work done Mass Acceleration Distance move
d
d (J)
Work done m/s m
Work done
10 30
147
2
.
Power
Work done
Time taken
W
but Work done Force Distance m
)
o
oved (J)
and Force Mass Acceleration (N)
By substitution,
Power
Mass Acceleration Distance moved
Time taken
W
Power
( )
10
0 30
60
2452 5
2
kg m/s m
s
Power W
.45kW.
.81m/s2
. This commemorates the name James Watt (1736–1819), the inventor
of the steam engine.
Definition
By definition, power the rate of
doing work.
Work done Force Distance moved the direction the force (J)
The unit work done the newton metre joule (symbol J).Basic Electrical Installation Work
88
Definition
By definition, work done dependent
upon the force applied times the dis-
tance moved the direction the
force. can given
amount work more quickly.
Example
A building hoist lifts ten 50kg bags cement through vertical distance 30m the top of
a high-rise building. definition, power the rate doing work. Calculate the work done the hoist, assuming the acceleration due to
gravity 9.
By definition Work done dependent upon the force applied times the
distance moved the direction the force.
The rating the building hoist motor will 2.
. Calculate
the rating (power) the motor perform this task 60s the acceleration due gravity is
taken 9.
Power
Work done
Time taken
W
)
The unit power, both electrical and mechanical, the watt (symbol
W).1
15kJ
Power
If one motor car can cover the distance between two points more quickly
than another car, say that the faster car more powerful. The joule
is the preferred unit and commemorates English physicist, James
Prescot Joule (1818–1889)
