题目

阅读理解Childhood is a time of fun and games. Many people still remember when they were young, they often played games like hide-and-seek and chess. Playing teaches children how to get with others, and it also helps to exercise the mind and body. However, children today spend most of their time in front of the computer, playing video games. A recent survey found that in the US. Children between 8 and 12 spend at least 13 hours a week playing video games or gaming. Boys of this age spend even more time, almost 16 hours a week. Although some people will say that gaming is helpful, it brings空间存在垂直于纸面方向的均匀磁场其方向随时间作周期性变化，磁感应强度B随时间变化的图线如图（甲）所示。规定B>0，时，磁场方向穿出纸面。现在磁场区域中建立一与磁场方向垂直的平面直角坐标，如图（乙）所示。一电量C质量kg的带正电粒子，位于原点O处，在时刻以初速度m/s沿轴正方向开始运动，不计重力作用，不计磁场变化可能产生的一切其他影响。 求：    （1）带电粒子的运动半径；    （2）带电粒子从O点运动到点的最短时间；    （3）要使带电粒子过图中的点，则磁场的变化周期为多少？