如图所示，一条长为L的细线，上端固定，将它置于一充满匀强电场的空间中，场强大小为E，方向水平向右。已知当细线向右偏离竖直方向的偏角为θ时，带电小球处于平衡状态。求：
⑴小球带电量为多少？
⑵如果使细线向右与竖直方向的偏角由θ增大为β，且自由释放小球，则β为多大时，才能使细线达到竖直位置时，小球的速度又刚好为零？
⑶如果将小球向左方拉成水平，此时线被拉直，自由释放小球后，经多长时间细线又被拉直？
（1）
（2）β=2θ （3）
题目分析：
（1）小球平衡状态 Eq=mgtanθ,解得
（2）对整个过程由动能定理 mgL(1-cosβ)-EqLsin β="0" ，解得 tanβ/2=tanθ,解得：β=2θ
（3）自由释放后小球作匀加速直线运动到细线又被拉直过程中， 因为 s = 2Lsinθ
而
根据
得
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| Learning Chinese may seem difficult to foreigners outside China. However, Daisy Raffan, an 1 girl in Britain, hopes to bring the 2 to the young people. Daisy has 3 an online learning website to teach the language to as 4 people as possible, all over the world. With China's fast development in the world, Daisy thinks 5 important for people to learn the language sooner rather than later. Daisy has been learning Putonghua 6 the age of five. Two years ago, 7 she decided to pass on what she had learned, she 8 the idea for the website, kidschineseclub.com. She said: "People are afraid to learn Putonghua. One of the main reasons is that they think it's an 9 language to learn, but I have had fun 10 it. And I hope what I have done is easy and fun." Her 11 offers a series of lessons, which are videos of Daisy teaching a group of students on topics from greetings and numbers to Chinese culture. Daisy's website has 12 the support of several important persons-the teenager says she has received a letter praising (赞扬) her work from Gordon Brown, 13 is the British Prime Minister (首相). She said: "I wrote to him and told him what I was doing and got a letter 14 . He said that what I was doing was a really good idea. I thought it was amazing. I was excited that 15 was supporting what we're doing." | ||||||
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