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Power
Average power:
ave
W
P
t
=
Δ
Instantaneous power:
t
dW
PF
dt
F
υ
===
i
G
G
P
=
F
υ
if force and velocity have the same direction,
P
=
−
F
υ
if they have opposite directions.
Example
The 5 Kg cart slides up the frictionless 30
°
ramp under the action of a horizontally applied force
F
as shown. The cart starts
from rest and moves with a constant acceleration of 0.5 m/s
2
from
A
to
B
. Find the average power by
F
for the motion from
A
to
B
. Find the instantaneous power of
F
when the cart reaches point
B
. (
Ans
:
P
ave
= 27 W,
P
B
= 54 W)
4 m
F
A
B
Workkinetic energy equation
2
11
2
2
1
1
2
2
KW K
m W
m
2
2
2
−−
+=
⇒
:
Motion of the body is from position 1 to position 2
K
= (1/2)
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 Fall '08
 TSChang
 Physics, Force, Friction, Power

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