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      Question

      A body is projected vertically upwards with a velocity

      of 98 m/s. The time interval during which it is at a height greater than 392 m is: (Take g = 9.8 m/s┬▓)
      A 4 s Correct Answer Incorrect Answer
      B 6 s Correct Answer Incorrect Answer
      C 8 s Correct Answer Incorrect Answer
      D 2 s Correct Answer Incorrect Answer

      Solution

      Maximum height H = u┬▓/2g = 98┬▓/(2 ├Ч 9.8) = 490 m Using the equation h = ut тИТ (1/2)gt┬▓, set h = 392 Solve 98t тИТ 4.9t┬▓ = 392 тЗТ 4.9t┬▓ тИТ 98t + 392 = 0 Solving the quadratic gives roots tтВБ = 4s, tтВВ = 12s So, height > 392 m for time interval t = 12 тИТ 4 = 8s.

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