Railways NTPC (Technical Ability) Engineering Mechanics and Strength of Materials Question Bank Engineering Mechanics

  • question_answer
    A spring of stiffness k is compressed by a length 'A' and a ball is placed on the top of it. When the spring is released the ball is seen to rise through a height h. When two such springs are kept one over the other, and the combination is compressed by same length X what is the height reached by the ball, kept on the combined springs when released?

    A) h/2                               

    B) h

    C) \[\sqrt{2}h\]                              

    D) 2h

    Correct Answer: A

    Solution :

    \[W=k\delta \] \[U=\frac{1}{2}k\delta \times \delta =\frac{1}{2}=k{{\delta }^{2}}=mgh\] \[m=\frac{k{{\delta }^{2}}}{gh}\] For two springs in series, \[{{k}_{e}}=\frac{k}{2}\]\[U=\frac{1}{2}\times \frac{k}{2}\times {{\delta }^{2}}=\frac{k{{\delta }^{2}}}{4}=mg{{h}_{1}}\] \[{{h}_{1}}=\frac{k{{\delta }^{2}}}{4mg}=\frac{k{{\delta }^{2}}}{4\times \frac{k{{\delta }^{2}}}{2h}}=\frac{h}{2}\]


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