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2. motion in straight line — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysics2. motion in straight line1 Mark
MCQ
A bullet from a gun is fired on a rectangular wooden block with velocity $u$.When bullet travels $24\,cm$ through the block along its length horizontally, velocity of bullet becomes $\frac{u}{3}$. Then it further penetrates into the block in the same direction before coming to rest exactly at the other end of the block. The total length of the block is $........\,cm$
  • A
    $30$
  • ✓
    $27$
  • C
    $24$
  • D
    $28$
✓

Answer

Correct option: B.
$27$
b
By $v^2=u^2+2 a s$

$\left(\frac{u}{3}\right)^2=u^2-2 a x$

$2 a x=u^2-\frac{u^2}{9}$

$2 a x=\frac{8 u^2}{9}.........(1)$

Similarly from starting

$v^2=u^2+2 ax$

$0= u ^2-2 ax _2$

$2 ax _2= u ^2..........(2)$

$By (1) /(2)$

$\frac{ x }{ x _2}=\frac{8}{9}$

$\frac{24}{ x _2}=\frac{8}{9}$

$x _2=27\,cm$

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