质量分别为2 kg和3 kg的两个小球，在光滑水平面上沿同一直线相向运动，速度大小分别为4 m/s和2 m/s。若碰撞后粘在一起，求碰后共同速度的大小。
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Correct answer
A. 0.4 m/s
Principle or equation
动量守恒定律：m1v1 + m2v2 = (m1+m2)v。规定正方向后代入。
Why this answer is correct
取2 kg球速度方向为正，则总动量 p = 2×4 + 3×(-2) = 8 - 6 = 2 kg·m/s；碰后共同速度 v = p/(2+3) = 2/5 = 0.4 m/s，方向与正方向相同。
Example
若m1=1kg，v1=3m/s，m2=2kg，v2=-1m/s，则v=(3-2)/3=1/3 m/s。
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