A particle moves along a straight line so that its displacement s (in metres) at time t (in seconds) is s = t³ - 6t² + 9t + 4. What is the velocity of the particle when its acceleration becomes zero?

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• A 0 m/s
• B –3 m/s
• C 3 m/s
• D –6 m/s

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Answer: B. –3 m/s

v = ds/dt = 3t² - 12t + 9 and a = dv/dt = 6t - 12.
a = 0 when t = 2 s.
v = 3(4) - 12(2) + 9 = 12 - 24 + 9 = -3 m/s.

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