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

  • question_answer
    The mass of balloon and its contents is M It is descending with an acceleration a. By how much the mass should be decreased, keeping the volume constant, so that the balloon starts ascending with the same acceleration?

    A)  \[\frac{a}{a+g}M\]        

    B)         \[\frac{g}{a+g}M\]

    C)  \[\frac{2a}{a+g}M\]        

    D)         \[\frac{2g}{a+g}M\]

    Correct Answer: C

    Solution :

     The forces acting on the balloon when the balloon is coming downwards with an acceleration \[a\] are  (i) weight, \[mg\] acting downwards and (ii) force of upthrust, \[F\] acting upwards. Since the accelerated movement is in downward direction, the force Mg must be greater than\[F\]. So for the downwards accelerated movement of the block,\[Mg-F=Ma\]                                                                                                                ... (i) Let \[m\] be the mass removed from the balloon, the weight of the balloon now becomes\[(M-m)g\]. Now for the upward accelerated movement with same acceleration, we have\[F-(M-m)g=(M-m)\]               ... (ii) Solving equations, (0 and (ii) the correct option can be seen to be [c].


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