Power
work done
time required
P
Fd
t
P
Fv
550
Energy, Work, and Power
POWER
Rev. 0
Page 9
CP-05
Mechanical Power
Mechanical energy used per unit time is called mechanical power. Mechanical power is the rate at
which work is done. Mechanical power is expressed in units of joules/sec (joules/s) or a watt (W)
in the mks system, and feet - pounds force per second (ft-lbf/s) or horsepower (hp) in the English
system. Mechanical power can be calculated using the following mathematical expression.
Because work can be defined as force times distance, we can also use the following equation:
(5-6)
where:
P
=
Power (W or ft-lbf/s)
F
=
Force (N or lbf)
d
=
distance (m or ft)
t
=
time (sec)
One horsepower is equivalent to 550 ft-lbf/s and 745.7 watts. Because in the equation above d
divided by t is the same as velocity, an alternate description of power is as follows.
(5-7)
where:
P
=
power (hp)
F
=
force (lbf)
v
=
velocity (ft/s)
When using equations 5-6 or 5-7, you must either assume force and velocity are constant or that
average values of the force and velocity are used.
