Question

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

• A. 1 s
• B. 2 s
• C. 3 s
• D. 4 s

Answer 1

Answer: (B)

Let the two balls cross each other after time $t$.
Height covered by first ball
$h_{1}=\frac{1}{2} g t^{2}$
Height covered by second ball
$h_{2}=u t-\frac{1}{2} g t^{2}$
but $h_{1}+h_{2}=100$
$\frac{1}{2} g t^{2}+u t-\frac{1}{2} g t^{2}=100$
$u t=100$
$50 \times t=100$
$\Rightarrow t=2\, s$