A body dropped from top of a tower covers a distance 7 x in last second of its journey, where x is d

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2.Motion in Straight Line

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A body dropped from the top of a tower covers a distance $7 x$ in the last second of its journey, where $x$ is the distance covered in first second.$.........\,s$ time does it take to reach the ground.

A

$3$

B

$4$

C

$5$

D

$6$

Solution

(b)

In first second distance travelled $x=\frac{1}{2} \times g \times t^2=5 m$ (for $g=10\,m / s ^2$ ) in last second $7 x=35\,m$

Now, $\quad s_t$ th $=\left(u+a t-\frac{1}{2} a\right)$

or $35=0+10 \times t-\frac{1}{2} \times 10$

or $\quad t=4\,s$

Standard 11

Physics

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