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49. No external torques act on the system consisting of the train and wheel, so the total
angular momentum of the system (which is initially zero) remains zero. Let
I = MR
2
be
the rotational inertia of the wheel. Its final angular momentum is
L
f
→
==
−
IM
R
ωω
##
kk
,
2
where
#
k
is
up
in Fig. 1147 and that last step (with the minus sign) is done in recognition
that the wheel’s clockwise rotation implies a negative value for
ω
. The linear speed of a
point on the track is
R
and the speed of the train (going counterclockwise in Fig. 1147
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 Spring '08
 Any
 Physics, Angular Momentum, Inertia, Momentum

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