Answer on Question #46908, Physics, Mechanics | Kinematics | Dynamics
A boy throws a ball vertically upward with a speed of 40 m/s and it reaches the maximum height H,
If he throws the same ball vertically downward from a height of H with initial speed of 40m/s, The
ball will reach ground with speed nearly:
The maximum height H is:
𝑣02
𝐻=
2𝑔
If the ball will be thrown with initial speed then:
𝑔𝑡 2
𝐻 = 𝑣0 𝑡 +
2
Where 𝑡 =
𝑣𝑒𝑛𝑑 −𝑣0
𝑔
𝐻 = 𝑣0
𝑣𝑒𝑛𝑑 − 𝑣0
+
𝑔
𝑣𝑒𝑛𝑑 − 𝑣0 2
𝑔(
)
𝑔
2
=
𝑣02
2𝑔
2𝑣0 (𝑣𝑒𝑛𝑑 − 𝑣0 ) + (𝑣𝑒𝑛𝑑 − 𝑣0 )2 = 𝑣02
(𝑣𝑒𝑛𝑑 − 𝑣0 )2 + 2𝑣0 (𝑣𝑒𝑛𝑑 − 𝑣0 ) − 𝑣02 = 0
(𝑣𝑒𝑛𝑑 − 𝑣0 + 𝑣0 )2 − 2𝑣02 = 0
(𝑣𝑒𝑛𝑑 )2 = 2𝑣02
𝑣𝑒𝑛𝑑 = 𝑣0 √2
𝑣𝑒𝑛𝑑 = 40
𝑚
𝑚
√2 ≈ 57
𝑠
𝑠
Answer: The ball will reach ground with speed nearly 𝑣𝑒𝑛𝑑 ≈ 57
𝑚
𝑠
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