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A ball is thrown vertically upwards from the ground It crosses a point at the height of `25 m` twice at an interval of `4 secs`. The ball was thrown with the velocity of.A. `20m//s`B. `25m//s`C. `40m//s`D. `30m//s` |
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Answer» Correct Answer - D A ball is ……….. `S = 25m`, u = ? `t_(Q) = t_(1) =4s` `a = 10m//s^(2)` `S = ut + (1)/(2)at^(2)` `25 = ut + (1)/(2) (-10)t^(2)` `5t^(2) - ut+25=0` `t_(1) +t_(2) = (u)/(5)` `t_(1)t_(2) =5` now `t_(Q) - t_(1) = sqrt((t_(1)+t_(2))^(2) -4t_(1)t_(2))` `4 = sqrt(((u)/(5))^(2) - 4xx5)` `u = 30m//s` |
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