1
Physics 2A
Lecture 16: Feb 13
Vivek Sharma
UCSD Physics
Force & Power
Work done is force applied over a distance. No mention of time in def.
ave
0
Power is time rate at which work is done. When a work W is done over time t
define average Power
and Instantaneous pow
WW
PP
=
l
i
m
er
tt
t
dW
dt
Δ→
Δ
Δ
Δ
ΔΔ
==
Δ
36
SI Unit of power: 1W = 1 J/s
More useful unit is KW=10 W or MW=10 W
1 hp = 550ft.lb/s= 746W
Unit of power can be used to define new unit of Work or Energy
Kilowatthour (kWh)

ave

ave
F
Δ
s
Δ
W
And finally,
P =
=
= F .
Δ
V
Δ
t
Δ
t
P
= F.v
G
G
Power !
(Girl)
Red Rocks, Nevada
Woman, m=50kg, climbs up
cliff h=400m in 15 minutes ! What’s
her power output in Watts & hp?
ave
Work Done
Δ
W
P
=
=
Time Taken
Δ
t
2
5
ave
=(50.0kg)(9.8m/s )(400m)
Δ
W
Δ
W=1.96×10 J & P =
=0.218kW
Δ
t
Since 1hp = 0.746kW
Δ
W = F.d = (mg)h
Girl power=0.29hp
⇒
⇒
G
G
Conservative Force
Work done by a conservative
force on an object moving from
one point to another
depends
only
only
on the initial and final
positions and is independent of
path taken.
e.g:
Gravity
2
1
2
1
2
Gg
1
G
2
1
W=F
.
co
s
θ
Use
cos
cos
()
W
y
y
dl
mg
dl
dy
dl
dl
mg
mg y
mg
dy
yh
φθ
−−
=
−
⇒=
−
=
=−
∫∫
∫
G
G
Conservative Force: Alternate Def.
A force is a conservative force IF the
net
work
done by the force on the object moving around
any
closed
path is zero
Work Done By Conservative Forces
Work done by
conservative force
:
1. Can be expressed as difference between initial and final values
of the potential energy functions
2.
It is reversible
3. Independent of path of body, depends on start/end point only
4.
When start/end point are same, work done is zero
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NonConservative Force
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 Winter '07
 Hicks
 Force, Potential Energy, Power, Work, Conservation of Mechanical Energy

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