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2.Motion in Straight Line
medium
$Assertion$ : If a body is thrown upwards, the distance covered by it in the last second of upward motion is about $5\, m$ irrespective of its initial speed
$Reason$ : The distance covered in the last second of upward motion is equal to that covered in the first second of downward motion when the particle is dropped.
AIf both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
BIf both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
CIf the Assertion is correct but Reason is incorrect.
DIf both the Assertion and Reason are incorrect.
(AIIMS-2000)
Solution
$\begin{array}{l}
For\,the\,{\rm{distance}}\,{\rm{covered}}\,{\rm{in}}\,{\rm{the}}\,{\rm{last}}\,\\
{\rm{second}},\,final\,velocity\,brcomes\,zero.\\
So\,if\,we\,drop\,an\,object\,with\,zero\,\\
velocity\,it\,will\,{\rm{cover}}\,{\rm{the}}\,{\rm{same}}\,{\rm{distance}}\\
{\rm{in}}\,{\rm{one}}\,{\rm{second}}\,{\rm{while}}\,{\rm{going}}\,{\rm{downwards}}{\rm{.}}\\
{\rm{now}}\,{\rm{distance}}\,{\rm{travelled}}\,{\rm{in}}\,{\rm{the}}\,{\rm{later}}\,{\rm{case}}\\
{\rm{s}}\, = ut + \frac{1}{2}g{t^2} = 0 + \frac{1}{2} \times 10 \times 1\\
s\, = 5m
\end{array}$
For\,the\,{\rm{distance}}\,{\rm{covered}}\,{\rm{in}}\,{\rm{the}}\,{\rm{last}}\,\\
{\rm{second}},\,final\,velocity\,brcomes\,zero.\\
So\,if\,we\,drop\,an\,object\,with\,zero\,\\
velocity\,it\,will\,{\rm{cover}}\,{\rm{the}}\,{\rm{same}}\,{\rm{distance}}\\
{\rm{in}}\,{\rm{one}}\,{\rm{second}}\,{\rm{while}}\,{\rm{going}}\,{\rm{downwards}}{\rm{.}}\\
{\rm{now}}\,{\rm{distance}}\,{\rm{travelled}}\,{\rm{in}}\,{\rm{the}}\,{\rm{later}}\,{\rm{case}}\\
{\rm{s}}\, = ut + \frac{1}{2}g{t^2} = 0 + \frac{1}{2} \times 10 \times 1\\
s\, = 5m
\end{array}$
Standard 11
Physics
