如图所示，物块M在静止的足够长的传送带上以速度v匀速下滑时，传送带突然启动，方向如图中箭头所示，若传送带的速度大小也为v，则传送启动后
A．M下滑的速度不变
B．M静止在传送带上
C．M先向下减速运动，后向上加速，最后与传送带一起向上运动
D．M可能沿斜面向上运动
答案：A
题目分析：传送带静止时，物块沿斜面匀速下滑，根据受力平衡，有
，摩擦力为滑动摩擦力方向沿斜面向上，与重力沿斜面向下的分力等大反向。传送带突然顺时针转动时，物块相对于沿斜面向下运动，摩擦力仍是滑动摩擦力，方向与相对运动方向相反即沿斜面向上，即传送带启动前后摩擦力大小方向都没有变化，所以物块仍保持匀速直线运动状态选项A对BCD错。
"Dear Grandpa Guo, we are losing our childhoods! Please help us!" Guo Chuanjie, a famous scientist, received a letter from a Beijing primary school student. The student said that many Chinese kids have to take extra classes on weekends in order to enter key middle schools. The kid hoped more people would hear his voice with Guo’s help. Guo is a member of the Chinese People’s Political Consultative Conference (CPPCC,中国人民政治 协商会议). Last month the CPPCC members met in Beijing. They are from all walks of life - scientists, businesspeople, doctors, artists and athletes. They give proposals (提案) on the country’s important issues(问题) to make our country better. For the Beijing student’s letter, Guo read it in front of Yuan Guiren, China’s Minister of Education (教育部部长), during a CPPCC meeting. According to Xinhua, there were 5,762 proposals in all. Many of the proposals were about problems like housing prices, heavy traffic and food safety, which were related to people’s daily lives. Some proposals were about teenagers. For example, singer Han Hong suggested that the government should deal with people who traffic(拐卖) children. As usual, proposals will be given to relative departments (相关部门) after the meetings. For example, Guo’s proposal was handed to the Ministry of Education. The departments will consider the proposals carefully and see if they can be carried out in the future. 小题1: The primary school student thought ____________ with Guo’s help.
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