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Chapter 7. Work and Kinetic Energy
Force
exerted through a
distance
performs
mechanical
work.
Which one costs energy?
1. Constant force in direction of motion.
W=F d
SI unit : Joule
1 J = N.m
Experiment on the front desk , Pull, rest, lift
My hand :
F= 15 N, distance = 2 m , W=30 J
2. Constant force at an angle
θ
to motion
W= F d cos
θ
Experiment on the front desk,
Pulling at
θ
=
20
,
F=15N, d=2 m
W= Fd cos
θ
=15*2* cos 20 = 28 J
Question: how to through a baseball to give it large speed?
Answer: Apply force across a large distance! (try it)
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What if
θ
=90 ?
cos
θ
= 0
W = 0
Normal force
What if
θ
=180 ?
cos
θ
= 1
W =  F d
kinetic Friction!
3. Negative Work:
when force and motion are in
opposite directions . 90 <
θ
≤
180
Examples: to catch a baseball, to stop a car
4, Total work:
Wtotal = W1 + W2 +W3+ ….
.
the sum of the work done by each forces.
Constant force at an angle
θ
to motion
W= F d cos
θθ
is angle between F & motion
The desk example again:
If μ
k
= 0.28,
μ
s
= 0.30,
mg = 49 N, if moved d=2m
My hand pull
F= 15 N
Pulling work:
W
Pull
= F . d = 15*2 =30 J
> 0
Table surface
f
k
=
μ
k
. N =
μ
k
mg = 0.28 x 49 =
13.7N
Friction work:
W
fk
= f
k
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 Spring '09
 JamesM.Lockhart
 Physics, Energy, Force, Kinetic Energy, Work, Work & Energy, total work

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