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Displacement, Velocity, and Acceleration

AP Physics 1· difficulty 1/5

A stone is dropped from rest from a 45 m45~\text{m} cliff. Using g≈10 m/s2g \approx 10~\text{m/s}^2 and ignoring air resistance, how long does it take to reach the ground?

  • A

    4 s4~\text{s}

  • B

    2 s2~\text{s}

  • C

    3 s3~\text{s}

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  • D

    4.5 s4.5~\text{s}

Explanation

From rest: h=12gt2h = \tfrac{1}{2}g t^2, so t=2h/g=2(45)/10=9=3 st = \sqrt{2h/g} = \sqrt{2(45)/10} = \sqrt{9} = 3~\text{s}.

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