Question

A ball is dropped from top of a tower of 100m height. Simultaneously another ball was thrown upward from bottom of the tower with a speed of 50 m/s ($g = 10m/s^{2})$ They will cross each other after

• A. $1s$
• B. $2s$
• C. $3s$
• D. $4s$

Answer 1

Answer: (B)

$2s$

Answer 2

$h_{1} = \frac{1}{2}gt^{2}$, $h_{2} = 50t - \frac{1}{2}gt^{2}$

Given $h_{1} + h_{2} = 100m$ ⇒ $50t = 100 \Rightarrow t = 2\mspace{6mu}\sec$