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1.wear down 磨损；使疲劳
The point of the pencil had been worn down when I finished this letter.
写完这封信时铅笔已经磨得很短了。
2.collide with 碰撞
Looping through the simulation allows the sphere to collide with the ground, and then come to rest.
仿真的循环执行是为了球体与地面相撞然后最终静止的整个过程。
