Arock thrown straight up takes 4.2 s to reach its maximum height what was its initial velocity

Consider the upward direction of motion as positive and downward direction of motion as negative.

a = acceleration due to gravity in downward direction = - 9.8

v₀ = initial velocity of rock in upward direction = ?

v = final velocity of rock at the highest point = 0

t = time to reach the maximum height = 4.2 sec

Using the kinematics equation

v = v₀ + a t

inserting the values

0 = v₀ + (- 9.8) (4.2)

v₀ = 41.2

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