题目

If you live in the United States, the law says you can't have a full­time job until you are 16 years old. At 14 or 15, you can work part-time after school or on weekends, and during summer you can work 40 hours each week. Does all that mean that if you are younger than 14, you can't earn(赚) your own money? Of course not! Kids who are between the ages of 10 and 13 can earn money by doing lots of things. Valerie, 11, told us that she made money by cleaning up other people's yards. Lena, 13, makes money by knitting(织) dog sweaters and selling them to her neighbors, and Riesa said sh如图所示是某同学探究加速度与力的关系的实验装置。他在气垫导轨上安装了一个光电门B，在小车上固定一遮光条，小车用细线绕过气垫导轨左端的定滑轮与钩码相连，每次小车都从A处由静止释放。(1)该同学用游标卡尺测量遮光条的宽度d如图所示，则d=______mm；(2)实验时，将滑块从A位置由静止释放，用游标卡尺测量遮光条的宽度d，由数字计时器读出遮光条通过光电门B的时间t，从刻度尺上得出AB之间的距离x，则小车的加速度a=_____（用给定的物理量表示）；(3)若实验没有满足钩码质量远小于小车质量，则会增大实验的____（填“偶然”或“系统”）误差；(4)在(3)的基础上做实验，改变小车质量M，钩码质量m不变，通过描点作出–M图象如图所示，已知图象斜率为k，纵轴截距为b，则m=_______。