15.一质点在 (0-xy 平面上运动,运动方程为: =0.2cos 2pi t-|||-0,则 t=0.5s 时刻质点的速度和加速度大小分别为() ()-|||-4 pi /s , .8pi m/(s)^2-|||-.4pi /s, .8pi m/(s)^2-|||-/s, .8pi m/(s)^2-|||-5 0m/s . .8(pi )^2m/(s)^2