- #1
KoaDcT
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Homework Statement
The problem is pretty simple, however I don't understand which value to use after using quad formula to solve. See below.
V1(initial) = 2.5 m/s
V2(Initial) = -5.9 m/s
A pool ball moving with a speed of 2.5 m/s makes an elastic head-on collision with an identical ball traveling in the opposite direction with a speed of 5.9 m/s. Find the velocities of the balls after the collision.
Homework Equations
ΣP(initial) = ΣP(final)
M1V1 + M2V2 = M1V1' + M2V2'
M1 = M2
2.5 + (-5.9) = V1' + V2'
V2' = -V1' - 3.4
Elastic collision: KE(initial) = KE(final)
(Mass cancels out)
V12+V22 = V1'2+V2'2
The Attempt at a Solution
From above, I got:
0 = 2V1'2 - 6.8V1' - 29.5
by substituting "V2' = -V1' - 3.4" for V2'
After applying quadratic equation I got:
V1' = -2.5 m/s, 5.9 m/s
My problem is I do not know which one is the correct answer. My answer key says V1' is equal to -2.5 m/s, but I have no idea why.
Any help is appreciated!