（2）從P到O沿直線必為勻速直線運(yùn)動(dòng)，設(shè)油滴質(zhì)量為m，由平衡條件有：

qvBsin37º＝qE                                              ……2分

mgtan37 º＝qE                                            ……2分

25．【答案】（1）油滴帶負(fù)電；（2）；（3）＋。

【解析】（1）油滴帶負(fù)電。油滴受重力、電場(chǎng)力和洛倫茲力三力作用，必做勻速直線運(yùn)動(dòng)，三力的合力為零，受力如圖，油滴只能帶負(fù)電。                                          ……4分

所以，要使得物塊不滑出平板車，平板車長(zhǎng)度至少為1.6m。                                 ……1分

解得：l＝1.6m。                                                                                                    ……2分

                                                                                            ……4分

解得：μ＝0.5。                                                                                                     ……2分

（3）物塊多次與障礙物碰撞后，最終與平板車同時(shí)停止。設(shè)物塊在平板車上運(yùn)動(dòng)的距離為l，根據(jù)功能關(guān)系有：

                                                                                                ……4分

24．【答案】（1）1m/s；（2）0.5；（3）1.6m。

【解析】（1）物塊與障礙物碰以后，物塊和小車系統(tǒng)動(dòng)量守恒，

有：Mv₀－mv₀＝(M＋m)v                                                                                            ……4分

解得：v＝1m/s。                                                                                                   ……2分

（2）物塊第一次與障礙物碰后向右減速到零時(shí)，向右運(yùn)動(dòng)的距離最在，對(duì)物塊，根據(jù)動(dòng)能定理有：

                                                                       ……3分

解得：v₀＝5m/s。                                                                                           ……1分

                                                                                                 ……2分

且有：F＝Mg                                                                                                   ……2分

對(duì)小球由a點(diǎn)運(yùn)動(dòng)到軌道中間位置的過(guò)程應(yīng)用動(dòng)能定理得：