通过限界检查必须在水平直线的轨道上进行。

下列变化既不需要加入还原剂，又不需要加氧化剂就能够实现的是

A .F₂→F^-        B. Cl₂→Cl^-           C .Mg→Mg²⁺        D. Na→Na⁺

图1为验证牛顿第二定律的实验装置示意图.图中打点计时器的电源为50Hz的交流电源，打点的时间间隔用Δt表示.在小车质量M未知的情况下，某同学设计了一种方法用来研究“在外力一定的条件下，物体的加速度与其质量间的关系”.

（1）完成下列实验步骤中的填空：

①平衡小车所受的阻力：撤去砂和砂桶，调整木板右端的高度，用手轻拨小车，直到打点计时器打出一系列     的点.

②按住小车，在左端挂上适当质量的砂和砂桶，在小车中放入砝码.

③打开打点计时器电源,释放小车,获得带有点迹的纸带,在纸带上标出小车中砝码的质量m.

④按住小车，改变小车中砝码的质量，重复步骤③.

⑤在每条纸带上清晰的部分，每5个间隔标注一个计数点.测量相邻计数点的间距s₁，s₂，….求出与不同m相对应的加速度a.

⑥以砝码的质量m为横坐标，为纵坐标，在坐标纸上做出-m关系图线.若加速度与小车和砝码的总质量成反比，则与m应成     关系（填“线性”或“非线性”）.

（2）完成下列填空：

①本实验中，为了保证在改变小车中砝码的质量时，小车所受的拉力近似不变，悬挂砂和砂桶的总质量应满足的条件是     .

②如图2所示是该同学在某次实验中利用打点计时器打出的一条纸带，A、B、C、D是该同学在纸带上选取的连续四个计数点.该同学用刻度尺测出AC间的距离为S_Ⅰ，测出BD间的距离为S_Ⅱ.a可用S_Ⅰ、S_Ⅱ和Δt（打点的时间间隔）表示为a=     .

③图3为所得实验图线的示意图.设图中直线的斜率为k，在纵轴上的截距为b，若牛顿定律成立，则小车受到的拉力为     ，小车的质量为     .

What is distance learning It means that you study on your own, at home or wherever suits you. Recently, the world famous Open University in the United Kingdom has designed a new style of distance learning, which is called (1) . The phrase "Open Learning" means you study (2) . You read course material, work on course activities, and write (3) . The word "Supported" means you have help (4) , the student services staff at regional centres, and centralized areas such as (5) . You can also contact other students through tutorials and (6) , the University’s online conferencing system, and events and clubs organized by (7) . Most distance learning courses use printed paper materials. They also include some (8) materials such as a CD, DVD or video. Many courses have a web site and an (9) . You’ll need access to a computer (10) to make use of these. The Open University can help its students buy a computer and (11) the cost of accessing the Internet.

With most distance learning courses, no (12) are required to study. of course, you have to be aged 18 when your course starts but there is no (13) . Currently the Open University has around (14) undergraduate and more than 30,000 postgraduate students, of which 10,000 have (15) . Nearly all students are studying (16) . About 70 per cent of undergraduate students are in (17) . More than 50,000 students (18) by their employers for their studies. Most distance learning courses (19) . Some of them are even available in other parts of the world. With over 25,000 of its students living outside the UK, the Open University is the (20) that offers distance learning throughout the world.

7（）

某人在高为h的山上将质量为m的石块以速度v₀斜向上方抛出，如图所示，v₀与水平方向的夹角为θ，不计空气阻力，则石块落到山脚时，速度大小的决定因素是

A．v₀和h          

B．v₀、θ和h

C．m、v₀和h      

D．m、v₀、θ和h

回流液的作用是什么？