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Question

A stone dropped from a certain height reaches ground in 5 sec. If it is dropped and stopped for infinitesimal time interval at t=3 s, then time taken by the stone to reach the ground is

A
6 s
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B
6.5 s
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C
7 s
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D
7.5 s
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Solution

The correct option is C 7 s

Given:
Under free fall, ball will reach ground in 5 s.
(Taking downward motion of ball as + ve)
s=ut+12at2
h=0+12gt2 =12×10×52=125 m

From t=0 to t=3 s.

Using,
s=ut+12at2
h1=0+12gt2 =12×10×32=45 m

At t=3 s ball is stoped for infinitesimal time interval which again makes the velocity of ball v=0 m/s.
As time interval is infinitesimal small,
we will neglect it while finding total time of fall.


h2=hh1=(12545) m=80 m
Again using,
s=ut2+12at22
h2=0+12gt22t2=2h2g s=4 s
Total time of fall=(3+4) s=7 s

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