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WAEC Further Mathematics 2006 Question 38

Question 38

Dynamics

A particle is projected vertically upwards from a height 45 metres above the ground with a velocity of 40ms−1. How long does it take to hit the ground? [Take g = 10ms−2]

  1. A.1s
  2. B.3s
  3. C.7s
  4. D.9s
Show the answer and explanation

Answer: D (9s)

Explanation

Taking up as positive, the ground is at −45 m: −45 = 40t − 5t². So t² − 8t − 9 = 0, (t − 9)(t + 1) = 0, and t = 9 s.