一物体从离地高20 m处由静止自由下落，不计空气阻力。求物体落地时的速度大小。（取g=10 m/s²）
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Correct answer
B. 20 m/s
Principle or equation
机械能守恒或自由落体运动学公式：v² = 2gh。
Why this answer is correct
由机械能守恒：mgh = (1/2)mv²，得 v = sqrt(2gh) = sqrt(2*10*20) = sqrt(400) = 20 m/s。
Example
若高度为5 m，则v = sqrt(2*10*5) = 10 m/s。
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