A stone projected upward from the top of a tower returns to the level of projection after 4 s and reaches the ground 2 s later. Taking g=10 m s⁻², the tower height and impact speed are respectively:
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Question

A stone projected upward from the top of a tower returns to the level of projection after 4 s and reaches the ground 2 s later. Taking g=10 m s⁻², the tower height and impact speed are respectively:

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G1 Physics · Accepted Answer

Returning to the launch level after 4 s gives 2u/g = 4 ⇒ u=20 m s⁻¹. The stone reaches the ground at total time 6 s. Its displacement from the tower top is y = 20(6) − 5(6²) = −60 m. Hence tower height = 60 m. Final velocity: v = 20−10(6) = −40 m s⁻¹. Impact speed = 40 m s⁻¹.