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JEE Mains · Physics · STD 11 - Work,energy,power and collision
  • JEE Mains 2019
  • Hard
  • Numerical

Three blocks \(A, B\) and \(C\) are lying on a smooth horizontal surface, as shown in the figure. \(A\) and \(B\) have equal masses, \(m\) while \(C\) has mass \(M\). Block \(A\) is given an initial speed \(v\) towards \(B\) due to which it collides with \(B\) perfectly inelastically. The combined mass collides with \(C\), also perfectly inelastically. The combined mass collides with \(C\), also perfectly inelastically if \(\frac{5}{6}^{th}\) of the initial kinetic energy is lost in whole process. What is value of \(M/m\) ?

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Answer & Solution

Correct Answer

(C) \(4\)

Step-by-step Solution

Detailed explanation

Apply \(LMC\) (Linear Momentum Conservation)

\(mv = (2m + M) v'\)

\(v' = \frac{{mv}}{{2m + M}}\)

Initial energy 

\(\frac{1}{2}\) \(m{v^2}\)

Final energy 

\(\frac{1}{2}\,\left( {2m + M} \right){\left( {\frac{{mv}}{{2m + M}}} \right)^2}\)

\(Initial\, kinetic\, energy - Final\, Kinetic\, energy =\)  \(\frac{5}{6}\) of initial kinetic energy.

After solving, we get, \(\frac{M}{m} = 4.\)

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