【简答题】Federal Child Labor Standards for agers age labor in the United States Federal Law sets child labor standards affecting age workers in the private companies and in federal, state, and loca...
【单选题】在光滑水平面上,有一轻弹簧,其一端固定,另一端连接一质量 m =1 kg 的滑块.弹簧自然长度 l 0 =0.4 m ,倔强系数 k =900 N × m - 1 .设 t =0 时,弹簧长度为 l 0 ,滑块速度大小 v 0 =5 m × s - 1 ,速度方向与弹簧垂直.在某一时刻,弹簧长度 l =0.5 m .则该时刻滑块速度方向与弹簧长度方向间的夹角为