An elevator ascends at \( 3 \, \text{m/s} \) for \( 5 \, \text{s} \), then accelerates at \( 2 \,
\text{m/s}^2 \) for \( 3 \, \text{s} \). A ball is dropped at the end of this period. What is the time
taken for the ball to hit the ground if the initial height was \( 20 \, \text{m} \)? (Take \( g = 10 \,
\text{m/s}^2 \))
Phase 1: \( h_1 = 3 \cdot 5 = 15 \, \text{m} \).
Phase 2: \( v = 3 + 2 \cdot 3 = 9 \, \text{m/s} \), \( h_2 = 3 \cdot 3 + \frac{1}{2} \cdot 2 \cdot (3)^2
= 9 + 9 = 18 \, \text{m} \).
Total height at drop = \( 20 + 15 + 18 = 53 \, \text{m} \), initial velocity of ball = \( 9 \, \text{m/s}
\) upward.
\( -53 = 9 t - 5 t^2 \Rightarrow 5 t^2 - 9 t - 53 = 0 \).
Solve: \( t = \frac{9 \pm \sqrt{81 + 1060}}{10} = \frac{9 \pm 33.79}{10} \), \( t = 4.28 \, \text{s} \)
(positive root).