Reviewed CSCA Physics question · Standard

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，求碰后它们的共同速度大小。

0.4 m/s
0.8 m/s
1.2 m/s
2.0 m/s

Show the answer and explanation

Correct answer

A. 0.4 m/s

Principle or equation

动量守恒定律：m1v1 + m2v2 = (m1+m2)v共（取正方向）。

Why this answer is correct

取第一个球运动方向为正，则v1=4 m/s，v2=-2 m/s。动量守恒：2*4 + 3*(-2) = (2+3)v，即8-6=5v，v=0.4 m/s。方向与第一个球相同。

Example

若m1=1 kg，v1=3 m/s，m2=2 kg，v2=-1 m/s，则v=(3-2)/3=1/3 m/s。

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