75. We choose
down
as the +
y
direction and use the equations of Table 21 (replacing
x
with
y
) with
a
= +
g
,
v
0
= 0 and
y
0
= 0. We use subscript 2 for the elevator reaching the
ground and 1 for the halfway point.
(a) Eq. 216,
vv
a
y
y
2
2
0
2
20
2
=+
−
bg
, leads to
()
2
22
2
2 9.8 m/s
120 m
48.5 m/s .
vg
y
==
=
(b) The time at which it strikes the ground is (using Eq. 215)
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 Spring '06
 Stoler
 Physics, m/s, halfway point

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