一个质量为1 kg的物体从离地面高5 m处由静止开始自由下落，不计空气阻力，重力加速度取10 m/s²。求物体落地时的速度大小。
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Correct answer
B. 10 m/s
Principle or equation
机械能守恒定律：只有重力做功时，动能和势能相互转化，总机械能不变。mgh = (1/2)mv²。
Why this answer is correct
由 mgh = 1/2 mv² 得 v = √(2gh) = √(2×10×5) = √100 = 10 m/s。
Example
若高度为20 m，则落地速度 v = √(2×10×20) = 20 m/s。
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