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9th Class Science Work and energy Question Bank Work, Energy and Power

  • question_answer
    A man throws the bricks to the height of 12 m where they reach with a speed of 12 m/s. If he throws the bricks such that they just reach this height, then what percentage of energy will he save?

    A)  19%                      

    B)         38%  

    C)  57%                      

    D)         76%

    Correct Answer: B

    Solution :

     In first case,\[{{W}_{1}}=\frac{1}{2}m{{(12)}^{2}}+mg\times 12\]                                   \[=\frac{1}{2}m\times 144+m\times 10\times 12\]                                   \[=192\,\,m\] In second case,                          \[{{W}_{2}}=mgh=\mathbf{120}\,\,\mathbf{m}\] \[\therefore \]\[%\]energy saved\[=\frac{192m-120m}{192m}\times 100%\] \[=\mathbf{38%}\]


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