新闻里多次报导家长抱孩子乘坐自动感应扶梯,因为受力变化而站立不稳.自动扶梯上没有人时静止,人踏上扶梯的水平踏板后,扶梯会自动以加速度a匀加速运动一段时间后再匀速运动.如图所示,质量为M的母亲抱着质量为m的婴儿踏上扶梯下楼,下楼过程中母婴始终保持与扶梯相对静止,设扶梯与水平面之间的夹角为θ.关于母婴受力情况分析正确的是() A . 电梯在匀速运动过程中,婴儿对母亲的作用力与扶梯运动方向一致 B . 电梯在匀速运动过
听对话和问题,根据所听内容,选择最佳答案。对话和问题都读两遍。
( )1. A. An English book. ( )2. A. yellow. ( )3. A. A ping-pong ball. ( )4. A. October 9th. ( )5. A. Lisa. ( )6. A. In the morning. ( )7. A. Jack. ( )8. A. 22. ( )9. A. Because it's interesting
B. A Chinese book. B. red B. A ping-pong bat B. September 19th. B. Anna. B. in the afternoon.B. Brown. B. 42. B. Science
C. A math book. C. blue C. A soccer ball C. October 19th. C. Lisa and Anna. C. in the evening.C. Jack Brown. C. 32. C. It's my book.