题目

（Madonna does it. Cameron Diaz and Reese Witherspoon do it. Jolin Tsai (Cai Yilin) also does it. But that's not why you should do yoga. The reason you should try this 5,000­year­old Indian practice is simple: It will help you reduce those extra pounds you packed on during the holiday season and make you a happier and hotter person. Yoga is a kind of mind­body exercise. The stretching washes away stress, anxiety, anger, and makes you feel two inches taller, tight and graceful. The word yoga means “union” in Sanskrit (梵语), the language of ancient India, where yoga originated. It ref如图所示，两平行导轨竖直放置，上端用导线相连，金属棒ab两端与导轨相接触，并可保持水平地沿光滑导轨滑动，整个处在方向垂直于导轨平面的匀强磁场中，导轨电阻忽略不计，已知金属棒电阻为0.5Ω，质量为0.5kg，ab长为25cm，B=2T（g取10m/s2），将金属棒由静止释放后，求：（1）刚释放时，金属棒的加速度；（2）金属棒下滑的最大速度vmax；（3）当速度为5m/s时，棒的加速度．