Many children act in TV shows. They work several hours every day, so they can not go to regular school. How do they get an education?

     In Hollywood, many TV shows are made. About forty teachers give lessons to the children in the shows. They teach wherever their pupils are working. The teachers’job is very important. They are responsible(负责) for making sure that the children work only the permitted(允许) hours each week. They are also responsible for making sure that children learn the required subjects. They make sure, too, that the children get enough rest and play, along with their education.

     Children actors are required to attend classes twenty hours each week. California law says they must be taught from September to June. If they do not make much progress in school, they are not permitted to continue working in TV shows. TV children are usually good pupils, and most of their teachers like this special kind of work. Their classes are held in some wonderful places. Sometimes the “classroom”is a Mississippi riverboat, and sometimes it is the inside of a spaceship. Often the pupils become famous TV or movie stars.

如图两段长均为L的轻质线共同系住一个质量为m的小球，另一端分别固定在等高的A、B两点，A、B两点间距也为L，今使小球在竖直平面内做圆周运动，当小球到达最高点时速率为v，两段线中张力恰好均为零，若小球到达最高点时速率为2v，则此时每段线中张力大小为（   ）