- #1
spacey__
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Homework Statement
"A falling object travels one-fourth of its total distance in the last second of its fall. From what height was it dropped?"
Reference: Up is (+)
Scenario 1 (Complete motion)
a = -9.8
v1 = 0
d = -x
v2 = ?
Scenario 2 (Motion at the last second)
a = -9.8
d' = (1/4)(-x)
t' = 1
v2 = ?
Homework Equations
Complete motion: v22 = v12 + 2ad
Motion at the last second: d' = v2t' - (1/2)(a)(t')2
3. The Attempt at a Solution
Setting both equation equal to v2 and then solving for x
Complete motion:
v22 = v12 + 2ad
v22 = (0)2 + (2)(-9.8)(-x)
v22 = (19.6)(x)
v2 = [(19.6)(x)](1/2)
Last second:
d' = v2t' - (1/2)(a)(t')2
(1/4)(-x) = v2 (1) - (-4.9)(1)2
Substitution:
(1/4)(-x) = [(19.6)(x)](1/2) - 4.9
[(1/4)(-x)]2 = (19.6)(x) + (-4.9)2
x2 / 16 = (19.6)(x) + (-4.9)2
x2 - (313.6)(x) - 384.16 = 0
x = - 1.22 x = 314.82
Edit: Was a problem with the signs should be
(1/4)(-x) = [(19.6)(x)](1/2) + 4.9
Answer is supposed to be x = 270
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