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9th Class Science Force and laws of motion Question Bank Force and Laws of Motion

  • question_answer
    A ball moving with a velocity u strikes a wall moving toward the ball with a velocity u. An elastic impact lasts for t sec. Then the mean elastic force acting on the ball is

    A)  \[\frac{2m\upsilon }{t}\]             

    B)         \[\frac{2m(\upsilon +u)}{t}\]  

    C)  \[\frac{2m(\upsilon +2u)}{t}\]                  

    D)  \[\frac{m(\upsilon +2u)}{t}\]

    Correct Answer: B

    Solution :

     Relative speed of the ball\[=(v+u)\] Speed after rebouncing\[=-(y+u)\]\[=-(v+u)\] Now,     \[F=m\frac{\Delta v}{\Delta t}=\frac{m}{t}[(v+u)]-[(v+u)]\]             \[=\frac{2m}{t}(v+u)\]


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