如图所示，平行金属板间有一静止的正电粒子，若两板间加电压u=Umsinωt，设板间距离足够大，则粒子的（ ）
A．位移一定按正弦规律变化
B．速度一定按正弦规律变化
C．加速度一定按正弦规律变化
D．粒子在两板间作单向直线运动
CD
题目分析：粒子的加速度为：
，即加速度一定按正弦规律变化，C正确；由于加速度方向发生变化则粒子将会做直线运动，其速度随时间变化的关系不按正弦规律变化；位移则一直朝一个方向。AB错误；D正确；
故选CD
点评：分析平行金属板间的电场强度随时间的变化情况，然后分析带电粒子所受电场力的情况，在电场力作用下所作的运动。
Today, there are many problems between parents and children. Many people always ask, “ Why do we often argue(争论)?” Parents show their love for children in different ways. They cook nice food, buy new clothes for them. They also care about their studies. Parents always ask “ You must work hard”, “You should do your homework”, “Why not do homework for your exam?” all the time. When children get bad marks(分数), their parents get very angry. They stop their children watching TV, playing games or going outside until they are good at their studies. Why? Most families have only one child. The child is the hope of the family, they want the child to be the best. Many children want to do things themselves. They want to do many unusual things that parents dislike. They like pop music, basketball and other new things. They enjoy surfing on the Internet and playing QQ games. They spend a lot of time chatting online and listening to the QQ music. In their parents’ eyes, these are a waste of time. For these reasons, parents and children often argue.
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