质量为2 kg的物体在水平面上受到大小为10 N的水平拉力作用，物体与水平面间的动摩擦因数为0.2，重力加速度取10 m/s²。求物体的加速度大小。
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Correct answer
A. 3 m/s²
Principle or equation
牛顿第二定律：F合 = ma。摩擦力 f = μmg，方向与运动方向相反。
Why this answer is correct
摩擦力 f = 0.2 × 2 × 10 = 4 N。合力 F合 = 10 - 4 = 6 N。加速度 a = F合/m = 6/2 = 3 m/s²。
Example
若拉力为12 N，则合力为8 N，加速度为4 m/s²。
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