Question

A stone is dropped from a bridge at a height of $180\, m$ over a river. After $3$ second, a second ball is thrown straight downwards. What should be the initial velocity of the second ball so that both hit the water simultaneously?

• A. 45 m / s
• B. 46 m / s
• C. 50 m / s
• D. 55 m / s

Answer 1

Answer: (A)

When stone is dropped $S =\frac{1}{2} gt ^{2}$
$180=\frac{1}{2} \times 10 \times t ^{2}$
$t=6$ seconds
Second ball is taking $3$ second to reach the river
$180= u \times 3+\frac{1}{2} \times 10 \times 9 \Rightarrow 180= u \times 3+5 \times 9 $
$u =\frac{180-45}{3}=45 m / s $