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WAEC Further Mathematics 2013 Question 16

Question 16

Dynamics

An object is thrown vertically upwards from the top of a cliff with a velocity of 25 ms−1. Find the time in seconds, when it is 20 meters above the cliff. [Take g = 10 ms−2]

  1. A.0 and 1
  2. B.0 and 4
  3. C.0 and 5
  4. D.1 and 4
Show the answer and explanation

Answer: D (1 and 4)

Explanation

20 = 25t − 5t², so t² − 5t + 4 = 0 and t = 1 or 4 (going up, then coming down).