Q. A ball falls from height $h$. After 1 second, another ball falls freely from a point $20\ m$ below the point from where the first ball falls. Both of them reach the ground at the same time. What is the value of $h$
A) $11.2\ m$
B) $21.2\ m$
C) $31.2\ m$
D) $41.2\ m$
Explanation
(c) For first ball $h = \frac{1}{2}gt^{2}$ …(i)
For second ball
$(h - 20) = \frac{1}{2}g(t - 1)^{2}$ …(ii)
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Subtract equation (ii) from (i)
$20 = - 5 + 10t \Rightarrow \ \therefore\ t = 2.5\ \sec$
Hence, height $h = \frac{1}{2} \times 10 \times (2.5)^{2} = 31.2\ m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.