From the top of an 80 m building, ball A is thrown vertically upward at 20 m s⁻¹ and ball B vertically downward at 10 m s⁻¹ at the same instant. Taking g = 10 m s⁻², the difference between their times of reaching the ground is:
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Question

From the top of an 80 m building, ball A is thrown vertically upward at 20 m s⁻¹ and ball B vertically downward at 10 m s⁻¹ at the same instant. Taking g = 10 m s⁻², the difference between their times of reaching the ground is:

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G1 Physics · Accepted Answer

For A, taking downward positive: 80 = −20t + 5t² ⇒ t² − 4t − 16 = 0, giving t_A = 2 + 2√5 ≈ 6.47 s. For B: 80 = 10t + 5t² ⇒ t² + 2t − 16 = 0, giving t_B = −1 + √17 ≈ 3.12 s. Difference ≈ 3.35 s, closest to 3 s.