Leon was a postman. Every day after Leon woke up, he would feed his 37 cats. Leon didn't really want 37 cats, but one by one they had turned up on his doorstep, and Leon had taken them in.

    Every day, after feeding the cats, Leon would collect the mail from the red postboxes in the village and take it home to sort（分类）. “If only I had 37 helpers instead of 37 cats, my job would be so much easier,” Leon thought. Just then, the cats started meowing for their dinner. That gave Leon an idea. That night, Leon stayed up making 37 cat-sized postal backpacks. Each backpack fitted neatly onto a cat's back. Then, he went around to each postbox, putting signs up that read: Postbox closed. Please use the Postal Cat Service.

    The next day, Leon put a backpack on each cat and dropped off the cats around the village. Leon knew that the cats would be happy lazing around all day. He also knew that they would always come home for dinner. Leon spent the day relaxing, waiting for his helpers to return. Leon was right. The cats did return, but they did not bring home many letters. Somehow, the cats had managed to wriggle（蜿蜒而行） out of their backpacks. Or lose their letters. Or get the letters wet. “Cats are just too unreliable（不可靠的）” Leon thought. Just then, there was a scratching（刮擦声） at Leon's door. A dog was waiting to be invited inside. That gave Leon an idea.

近代以来，为了民族的独立、国家的富强，无数中华儿女前赴后继，经历了一百多年的探索，给我们留下了宝贵的财富。阅读下列材料，回答问题。

材料一：李鸿章自称大清“裱糊匠”，他说：“我办了一辈子的事，练兵也，海军也，都是纸糊的老虎，何尝能实在放手办理？不过勉强涂饰，虚有其表，不揭破犹可敷衍一时……”

材料二：在这次上书中(上清帝第五书)，康有为围绕召开国会、定宪法的主张……（提出）皇帝仿效亚历山大二世和明治天皇，亲自主持大计，宣布变法……

材料三：辛亥革命作为中国近代史上的伟大事件，“无疑是中国近代史上的一次飞跃，它推翻了中国最后一个封建王朝，结束了在中国绵延几千年并被奉若神明的君主专制制度，破天荒地宣布了主权‘属于国民全体’，从而扫清了中国社会前进的巨大障碍，使民主共和的思想深入人心，其政治意义是决不能低估的”。

材料四：十月革命一声炮响，给我们送来了马克思主义。

材料五：英国《金融时报》：“自1978年以来，中国企业有三次创业的高潮点，分别是1984年、1992年和2001年。”

材料六：1990年，中央决定“以上海浦东开发开放为龙头，进一步开放长江沿岸城市”。2004年，中央“希望把上海建设成为国际经济、金融、贸易、航运中心和现代化大都市”。2013年9月29日，中国上海自贸区正式挂牌。

⑴下面是实验的主要步骤：

①安装好气垫导轨，调节气垫导轨的调节旋钮，使导轨水平；

②向气垫导轨通入压缩空气；

③接通光电计时器；

④把滑块2静止放在气垫导轨的中间；

⑤滑块1挤压导轨左端弹射架上的橡皮绳；

⑥释放滑块1，滑块1通过光电门1后与左侧固定弹簧的滑块2碰撞，碰后滑块1和滑块2依次通过光电门2，两滑块通过光电门后依次被制动；

⑦读出滑块通过两个光电门的挡光时间分别为滑块1通过光电门1的挡光时间△t₁=10.01ms，通过光电门2的挡光时间△t₂=49.99ms，滑块2通过光电门2的挡光时间△t₃=8.35ms；

⑧测出挡光片的宽度d=5mm，测得滑块1（包括撞针）的质量为m_l=300g，滑块2（包括弹簧）质量为m₂=200g；

⑵数据处理与实验结论：

①实验中气垫导轨的作用是____②碰撞前滑块1的速度v₁为____m/s；碰撞后滑块1的速度v₂为____m/s；滑块2的速度v₃为____ m/s；（结果保留两位有效数字）

③在误差允许的范围内，通过本实验，同学们可以探究出哪些物理量是不变的？通过对实验数据的分析说明理由．（至少回答2个不变量）．

a．____b．____．