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2.Motion in Straight Line
hard
A stone is dropped from the top of a building. When it crosses a point $5\, m$ below the top, another stone starts to fall from a point $25\, m$ below the top. Both stones reach the bottom of building simultaneously. The height of the building is ..... $m$.
A
$35$
B
$45$
C
$50$
D
$25$
(JEE MAIN-2021)
Solution
displacement of the first stone when second is dropped $=5 m$,
time difference in dropping of the stones is $t =\frac{2 h }{ g }=1\; s$
$h =\frac{1}{2} g t ^{2}=\frac{1}{2} g ( t -1)^{2}+25 \Rightarrow t =3 \;s$
thus, $h =\frac{1}{2} g t ^{2}=\frac{1}{2} \times 10 \times (3)^{2}= 4 5 \;m$
Standard 11
Physics
