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 
Answer:
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Explanation:
Answer:
Explanation:
Let the forward displacement is taken is positive, and the backward displacement is taken is negative.
first displacement = + 18 cm
second displacement = - 6 cm
third displacement = - 12 cm
net displacement = 18 - 12 - 6 = 0 cm