平板车以速度v向正东方向匀速运动，车上一物体在水平力F作用下相对车以速度v向
正南方向匀速运动（车速不变，物体没有脱离平板车）。物体质量m，物体与平板车间的
动摩擦因数为μ，重力加速度为g，则
A．F的大小为
μmg
B．F的大小为μmg
C．F的方向为东偏南45°
D．F的方向为正南
BD
题目分析：小车与物体均匀速，故物体受力平衡，水平方向只受摩擦力与拉力F，必等大反向。由于滑动摩擦力与相对运动方向相反，物体相对于车向正南运动，故车对其摩擦向正北，故拉力F向南，再根据摩擦力公式知摩擦力为μmg，故拉力F也为μmg，故选项BD正确。
| 阅读理解。 |
| When you are in England, you must be very careful in the street because the traffic drives on the left. You must look at the right first and then the left, then you can cross the street. If the traffic lights are red, the traffic must stop. Then the people on foot can cross the road. If the traffic lights are green, the traffic can go. People on foot mustn't cross. When you go by bus in England, you have to be careful, too. Always remember the traffic moves on the left. Have a look first, or you will go the wrong way. In many English cities, there are big buses with two floors. You can sit on the second floors. From there you can see the city very well. It's very interesting. |
| 1. When you are in England, you must remember that ______. |
[ ] |
| A. there are not any traffic lights. B. the traffic drives on the left. C. the traffic drives on the right. D. there are many cars on the road. |
| 2. Before you cross a street in Shanghai, you must look at ______ first and then ______. |
[ ] |
| A. the left, the right B. the right, the left C. the front, the black D. the back, the front |
| 3. Why must you be careful when you go by bus in England? |
[ ] |
| A. Because there are too many cars. B. Because you may go the wrong way. C. Because the traffic lights are different from ours. D. Because there are too many people. |
| 4. It's interesting to ______ in England. |
[ ] |
| A. look at the people in the street. B. cross the street. C. sit on the second floor of a big bus and see the city. D. look at the traffic lights. |
| 5. Which is the best title of this passage? |
[ ] |
| A. Traveling in England. B. Buses in England. C. Traffic in England. D. Traffic lights in England. |