Question

如圖（a）所示，在足夠長(zhǎng)的光滑水平面上，放置一長(zhǎng)為L＝1m、質(zhì)量為m₁＝0.5kg的木板A，一質(zhì)量為m₂＝1kg的小物體B以初速度v₀滑上A的上表面的同時(shí)對(duì)A施加一個(gè)水平向右的力F，A與B之間的動(dòng)摩擦因數(shù)為μ＝0.2，g＝10m/s²；小物體B在A上運(yùn)動(dòng)的路程S與F力的關(guān)系如圖（b）所示。求：v₀、F₁、F₂。

Answer 1

（1）4m/s；（2）3N；（3）9N

（1）由圖象可看出當(dāng)F≤1N時(shí)，B物體在A板上的路程始終等于板長(zhǎng)L，當(dāng)F＝1N時(shí)，剛好不從A板右端掉下，此后A和B一起相對(duì)靜止并加速運(yùn)動(dòng)。
設(shè)B物體的加速度為a₂，A板的加速度為a₁，分別由牛頓第二定律：
μm₂g＝m₂a₂ ············································································································ ①
F＋μm₂g＝m₁a₁ ······································································································· ②
設(shè)B運(yùn)動(dòng)的位移為S₂，A運(yùn)動(dòng)的位移為S₁，經(jīng)過t時(shí)間兩者速度均為v，根據(jù)運(yùn)動(dòng)學(xué)公式：
S_B＝t ············································································································· ③
S_A＝t ·················································································································· ④
v＝v₀－a₂t＝a₁t········································································································· ⑤
B在A上相對(duì)A向右運(yùn)動(dòng)的路程S＝S_B－S_A ······································································ ⑥
聯(lián)立①②③④⑤⑥解得：S＝··············································································· ⑦
將F＝1N，S＝1m代入，解得：v₀＝4m/s
（2）根據(jù)⑦式分析可知，當(dāng)1N≤F≤F₁時(shí)，隨著F力增大，S減小，當(dāng)F＝F₁時(shí)，出現(xiàn)S突變，說明此時(shí)A、B在達(dá)到共同速度后，恰好再次發(fā)生相對(duì)運(yùn)動(dòng)，B將會(huì)從A板左端掉下。
對(duì)A、B恰好發(fā)生相對(duì)運(yùn)動(dòng)時(shí)，B的加速度為a₂，則整體加速度也為a₂，由牛頓第二定律：
F₁＝(m₁＋m₂)a₂········································································································ ⑧
聯(lián)立①⑧解得解得F₁＝3N
（3）此時(shí)B在A上運(yùn)動(dòng)的路程為S₁＝＝m
當(dāng)F≥F₁時(shí)，物體B在A板上的路程為B相對(duì)A向右運(yùn)動(dòng)的路程的兩倍。
故當(dāng)F＝F₂時(shí)，將S＝0.5S₁代入⑦式解得：F₂＝9N
【評(píng)析】本題考查牛頓運(yùn)動(dòng)定律�；瑝K問題是物理模型中非常重要的模型，是學(xué)生物理建模能力培養(yǎng)的典型模型。滑塊問題的解決非常靈活，針對(duì)受力分析、運(yùn)動(dòng)分析以及牛頓第二定律的掌握，還有相對(duì)運(yùn)動(dòng)的分析，特別是摩擦力的變化與轉(zhuǎn)型，都是難點(diǎn)所在。本題通過非常規(guī)的圖象來分析滑塊的運(yùn)動(dòng)，能從圖中讀懂物體的運(yùn)動(dòng)。