【简答题】DJs (disc jockeys) are the people who play and present music on the radio, or in nightclub. A VJ (video jockey) is a person who introduces music videos on television. VJs were first seen on television...
【简答题】一弹簧振子沿 x 轴作简谐振动(弹簧为原长时振动物体的位置取作 x 轴原点).已知振动物体最大位移为 x m = 0.4 m 最大恢复力为 F m = 0.8 N ,最大速度为 u m = 0.8 p m/s ,又知 t = 0 的初位移为 + 0.2 m ,且初速度与所选 x 轴方向相反. (1) 求振动能量; (2) 求此振动的表达式.