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

  • question_answer
    A body of mass 2 kg is thrown up vertically with a kinetic energy of 490 J. If the acceleration due to gravity is \[9.8\text{ }m\text{ }{{s}^{-2}}\], the height at which the kinetic energy of the body becomes half of the original value is

    A) 50 m   

    B)        25 m   

    C)        12.5 m

    D)        10 m.

    Correct Answer: C

    Solution :

    We know, if a body is thrown up vertically its KE goes on decreasing and PE goes on increasing. There is conversion of KE into PE. When kinetic energy becomes half of original value, the other half is converted into potential energy. Therefore, \[\frac{\text{490}}{\text{2}}\text{=2 }\!\!\times\!\!\text{ 9}\text{.8 }\!\!\times\!\!\text{ h}\] Or \[\text{h=}\frac{490}{2\times 2\times 9.8}\] or \[h=12.5m\] Thus, the required height is \[12.5m\]


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