Physics 111.01
October 3, 2008
PS 5c (13)
Problem 1 = Walker P 708.
A child pulls a friend in
a little red wagon with constant speed. If the child pulls
with a force of 16 N for
14.4
m, and the handle of the
wagon is inclined at an angle of
28
° above the
horizontal, how much work does the child do on the
wagon?
F
m
θ
g
Solution.
"Work = force times distance."
This
means the component of the force in the direction
of the motion:
( )
cos
16 N cos28
14.13 N
Ff
θ
==
°
=
parallel to motion
Then the work is equal to
( )( )
14.13 N
14.4 m
203.4 J
WF
d
=
parallel to motion
Problem 2 = Walker P 718.
A
0.12
kg pinecone falls
15.5
m to the ground, where it lands with a speed of
16
m/s.
g
h
m
v
(a) With what speed would the pinecone have landed if
there had been no air resistance?
(b) Did air resistance do positive work, negative work,
or zero work on the pinecone?
Solution.
(a)
We work this with the new workinto
energy method.
2
1
K m
v
2
dm
g
h
Solving for the velocity gives
()
2
9.8 m/s
15.5 m
17.43 m/s
=
2
2
vg
h
(b)
Now, the actual speed was less, so the pine cone did not have as much
energy as it would have without air resistance.
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 Fall '08
 Staff
 Force, 2 K, 9.8 m/s, Wnet, 2 2 K

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