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Question

The blocks A and B, each of mass m, are connected by massless spring of natural length L and spring constant k. The blocks are initially resting on a smooth horizontal floor with the spring at its natural length, as shown in figure. A third identical block C, also of mass m, moves on the floor with a speed v along the line joining A and B, and collides with A. Then

  1. The KE o the A minus B system, at maximum compression of the spring, is zero    
  2. The KE of the A minus B system, at maximum compression of the spring is fraction numerator 1 over denominator 4 end fraction m v to the power of 2 end exponent    
  3. The maximum compression of the spring is v square root of fraction numerator m over denominator k end fraction end root    
  4. The maximum compression of the spring is v square root of fraction numerator m over denominator 2 k end fraction end root    

The correct answer is: The maximum compression of the spring is v square root of fraction numerator m over denominator 2 k end fraction end root


    The compression of spring is maximum when velocities of both blocks A and B is same. Let it be v subscript 0 end subscript, then from conservation law of momentum
    m v equals m v subscript 0 end subscript plus m v subscript 0 end subscript equals 2 m v subscript 0 end subscript rightwards double arrow v subscript 0 end subscript equals fraction numerator v over denominator 2 end fraction
    therefore kinetic energy of A minus B system at that stage
    equals fraction numerator 1 over denominator 2 end fraction open parentheses m plus m close parentheses cross times open parentheses fraction numerator v over denominator 2 end fraction close parentheses to the power of 2 end exponent equals fraction numerator m v to the power of 2 end exponent over denominator 4 end fraction
    Further loss in KE> = gain in elastic potential energy
    i e, fraction numerator 1 over denominator 2 end fraction m v to the power of 2 end exponent minus fraction numerator 1 over denominator 4 end fraction m v to the power of 2 end exponent equals fraction numerator 1 over denominator 4 end fraction m v to the power of 2 end exponent equals fraction numerator 1 over denominator 2 end fraction k x to the power of 2 end exponent
    rightwards double arrow blank x equals v square root of fraction numerator m over denominator 2 k end fraction end root

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