Television is good. You’d like to watch it all day and all night. But too much TV is a very bad thing. Scientists found that kids who watch too much TV may have more trouble learning to read. They can’t focus on their work.
   Kids learn language skills best by reading, talking and playing with others. If they spend too much time watching TV, they’ll have less time for those things. Watching too much TV is bad for people’s health. People don’t move much while watching TV. What’s more, they may eat a lot of food while watching. This can make them fat. We call these people couch potatoes.
   Fighting on TV is a big problem for kids. Yong people are good at following. Scientists have found that those who watch a lot of fighting shows are more likely to fight.

如图所示，两条互相平行的光滑金属导轨位于水平面内，距离为l=0.2m，在导轨的一端接有阻值为R=0.5Ω的电阻，在x≥0处有与水平面垂直的均匀磁场，磁感应强度B=0.5T．一质量为m=0.1kg的金属直杆垂直放置在导轨上，并以v₀=2m/s的初速度进入磁场，在安培力和一垂直于杆的水平外力F的共同作用下做匀变速直线运动，加速度大小为a=2m/s²，方向与初速度方向相反．设导轨和金属杆的电阻都可以忽略，且接触良好，求：