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Question
A ball is thrown vertically upward from the ground with an initial velocity of 39.2 m/sec. If the only force considered is that attributed to the acceleration due to gravity, find how long will it take for the ball to strike the ground?
Solution
Initial velocity of the ball = 39.2 m/sec
Let s be the distance of the ball from the ground at time t.
u = 39.2 m/s.
s = `"ut" - 1/2 * "gt"^2` where g = 9.8 m/sec.
s = `39.2 "t" - 1/2 xx (9.8)"t"^2`
s = 39.2 t – 49 t2
For the ball to strike the ground, s = 0
∴ 39.2t – 4.9t2 = 0
t(39.2 – 4.9t) = 0
39.2 – 4.9t = 0 since t ≠ 0
4.9t = 39.2
t = `39.28/4.9` = 8
t = 8 sec.
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