式中 s＝84 m。由①②③式得    ④

代入题给数据v_A=20m/s，v_B=4m/s，a =2m/s²，

                                       ③

                      ②

式中，t₀ =12s，s_A、s_B分别为 A、B两车相遇前行驶的路程。依题意有

                                             ①

15．   设A车的速度为v_A，B车加速行驶时间为t，两车在t₀时相遇。则有

                                                                   球体对斜面的压力的大小：

                                                                   方向垂直斜面向下

                                                                   球体对挡板的压力大小：

                                                                   方向水平向左

14．

                                                       如图小球受三个力作用（mg、F₁、F₂）

                                     由几何关系有