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79. The initial height of the 2
M
block, shown in Fig. 864, is the
y
= 0 level in our
computations of its value of
U
g
.
As that block drops, the spring stretches accordingly.
Also, the kinetic energy
K
sys
is evaluated for the
system
 that is, for a total moving mass
of 3
M
.
(a) The conservation of energy, Eq. 817, leads to
K
i
+ U
i
= K
sys
+ U
sys
¡
0 + 0 =
K
sys
+
(2
M
)
g
(–0.090) +
1
2
k
(0.090)
2
.
Thus, with
M
= 2.0 kg, we obtain
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 Spring '08
 Any
 Physics, Energy, Kinetic Energy

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