MCQ
$A$ block of mass $m$ starts from rest and slides down $a$ frictionless semi-circular track from $a$ height $h$ as shown. When it reaches the lowest point of the track, it collides with a stationary piece of putty also having mass $m$. If the block and the putty stick together and continue to slide, the maximum height that the block-putty system could reach is:
  • $h/4$
  • B
    $h/2$
  • C
    $h$
  • D
    independent of $h$

Answer

Correct option: A.
$h/4$
a
The velocity of block just before hitting the putty (by energy conservation)

$K . E_{i}+P . E_{i}=K . E_{f}+P . E_{f}$

$\Rightarrow 0+m g h=\frac{1}{2} m v^{2}$

$\Rightarrow v=\sqrt{2 g h}$

Let the common velocity of both block and putty be $v^{\prime}$

At collision momentum balance:

$m v=m v^{\prime}+m v^{\prime}$

$\Rightarrow \sqrt{2 g h}=2 v^{\prime}$

$\Rightarrow v^{\prime}=\sqrt{\frac{g h}{2}}$

Again applying energy conservation (both block and putty at max height)

$K . E_{i}+P . E_{i}=K . E_{f}+P . E_{f}$

$\Rightarrow \frac{1}{2} 2 m v^{\prime 2}=2 m g h^{\prime}$

$\Rightarrow \frac{1}{2} \frac{g h}{2}=g h^{\prime}$

$\Rightarrow h^{\prime}=h / 4$

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