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Vf ² = Vi ² + 2ad
Vf ² = 0 + 2(9.8)(20)
Vf ² = 392
Vf ~ 19.80 m/s when it lands.
Initial speed coming back up:
(3/4)(19.80) = 14.85 m/s
Equation of motion for second bounce:
d = Vit + (1/2)at ²
d = 14.85t + (1/2)(-9.8)t ²
Substitute d = 0 (to represent the landing height) and solve for t.