材料一：“丝绸之路经济带”和“21世纪海上丝绸之路”的战略构想，给古老的丝绸之路带来了新的外延与内涵，给中国以及寻求发展的沿途各国带来了新的机遇与未来。

材料二：世界某区域略图。

   People are often killed while c____  the road. Most of them are old people and c____ . Old people are often killed because they are careless. People should look and listen b____  they cross the road. A car, a truck or a bus can't stop very q____ if it is going very fast. It will travel many meters before it s____ . Pedestrian(行人)don't know this. They think that a car can stop right away. It is h____ for a pedestrian to know how fast a car is traveling. The only s____ way to cross the road is to look at b____ ways, right and left. The correct way to cross the road is t____  walk quickly when the traffic l____ turn green.

①如图1所示，先将沙和沙桶通过滑轮悬挂于小车一端，调节平板的倾角θ，使小车沿斜面向下做匀速直线运动，测出沙和沙桶的总质量m；

②保持平板倾角θ不变，去掉沙和沙桶，小车即在平板上沿斜面向下做匀加速直线运动，通过纸带测量其加速度a；

③保持小车质量M不变，多次改变沙和沙桶的总质量m，每次重复①②两步操作，得到小车加速度与合力的关系；

④多次改变小车的质量，进行适当的操作，得到小车加速度和质量的关系．