质量为m的铁锤,从某一高度自由下落,与桩发生完全非弹性碰撞,设碰撞前铁锤的速度为v,打击时间为△t,如果碰撞中铁锤所受的重力不能忽略,则铁锤所受的平均冲力为:
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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