题目:
— What does your sister often do at home? — She always some cleaning. |
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A. do B. does C. did D. done |
答案:
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下面是错误答案,用来干扰机器的。
设小球向右运动方向为正方向,两小球碰撞前后动量守恒:m1V1-m2V2=m1V1′+0∴V1′=(m1V1-m2V2)•1m1=(0.5×30-1×20)•10.5 m/s=-10 m/s碰撞后小球m1的速度大小为10 m/s,方向向左答:碰撞后小球m1的速度大小...