（15分）粗细均匀的直导线ab的两端悬挂在两根相同的弹簧下边，ab恰好处在水平位置（如图）。已知ab的质量m=0.5kg，ab的长度l=0.2m沿水平方向与ab垂直的匀强磁场的磁感应强度B="5T" 。g取10 m/s²。

（1）导线中未通电流时，弹簧伸长了0.05m，则弹簧的劲度系数是多少？

（2）要使两根弹簧能处于自然状态（即不被拉长，也不被压缩），ab中应沿什么方向、通过多大的电流？

（3）当由b到a方向通过2A的电流时，两根弹簧被拉长多少？

书面表达

     假设你是本刊读者Jack，请你根据所给的背景材料给Peter写封回信.内容包括：

1.分析Peter父母不同意的理由；

2.你给Peter的具体建议。

背景材料：

Teenagers          总228(4/2011)期

编者按:本期刊登了Peter的来信,欢迎读者就信中谈到的问题进行分析并提出建议.

Editor,

     I'm a boy of 18. I like arts, especially dancing. I want to attend a hip-hop(街舞) training course on the weekend, but my parents don't permit me to. I'm very upset and need your advice.

                                                                                                                                                        Peter

注意:字数100左右.文章的开头已经给出,不计入总字数.Peter,

     You are very upset that your parents don't permit you to attend a hip-hop training course on the

weekend._______________________________________________________________________________________________________________________________________________________.

                                                                                                                                                      Yours,

                                                                                                                                                       Jack