Answer:
11.2 m/s
Explanation:
We need to first figure out how long it took to reach the ball's max height:
Vf = 0
Vi = 30 m/s
a = -9.81 m/s^2
t = ?
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Vf = Vi + at
0 = 30 + (-9.81)t ---> -30 = -9.81t ---> 3.058 = t
t = 3.058 s
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We can now solve the problem by subtracting the given time (4.2s) by the max height time (3.058s) and plugging that into a kinematic equation for Vf:
Vi = 0
a = 9.81 m/s^2
t = (4.2s - 3.058s) = 1.142 s
Vf = ?
---------------
Vf = Vi + at
Vf = 0 + (9.81)*(1.142) ---> Vf = 11.203
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The velocity of the ball after 4.2s is 11.2 m/s
