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  4. 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 :

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Q. 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 :

JEE MainJEE Main 2021Motion in a Straight Line

Solution:

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Time for particle to meet $=t'=\frac{S_{\text {rel }}}{S_{\text {rel }}}$
$=\frac{20}{10}=2 sec$
Time taken by $I^{st}$ particle to reach ground $=3 sec$
$H =\frac{1}{2} g (3)^{2}=45 m$