问题 计算题

(13分)如图所示,质量为m=10kg的小物体在F=200N的水平推力作用下,从粗糙斜面的底端由静止开始沿足够长的斜面运动,斜面固定不动,小物体与斜面间的动摩擦因数μ=0.25,斜面与水平地面间的夹角θ=37°,力F作用t1=2s后撤去,小物体在斜面上继续上滑后,又返回至起点。(已知sin37°=0.6,cos37°=0.8,g取10m/s2)

⑴小物体运动过程中相对起点的最大位移xm的大小;

⑵小物体从开始经过多长时间t返回起点。

答案

⑴xm=16.25m;⑵t=s

题目分析:⑴当物体上滑至最高点,速度为零时,相对起点的位移最大,设在t1时间内物体运动的加速度为a1,撤去力F瞬间时物体的速度为v,撤去力F后物体向上滑行的加速度为a2,又经时间t2物体的速度减为零,在有力F作用时,根据牛顿第二定律有:Fcosθ-mgsinθ-μ(Fsinθ+mgcosθ)=ma1       ①

根据匀变速直线运动速度公式有:v=a1t1      ②

撤去力F,物体继续上滑的过程中,根据牛顿第二定律有:-mgsinθ-μmgcosθ=ma2            ③

根据匀变速直线运动速度公式有:v=-a2t2        ④

物体上滑的位移为:xm         ⑤

由①②③④⑤式联立,并代入数据解得:t2=1.25s,xm=16.25m          ⑥

⑵物体下滑过程中,根据牛顿第二定律有:mgsinθ-μmgcosθ=ma3         ⑦

设物体下滑的时间为t3,根据匀变速直线运动位移公式有:xm         ⑧

物体从开始到返回起点的过程中所经历的时间为:t=t1+t2+t3          ⑨

由⑥⑦⑧⑨式联立,并代入数据解得:t=s

完形填空

One’s education starts from family, especially from his or her mother. Here is a story__26__ happened in my childhood.
One Saturday morning, I was watching TV in the living room with my two brothers. My mum__27__ several apples of different colors and sizes. She put all the apples on the table, then __28__me and my younger brothers, “Which do you like ?”__29__ my younger brothers and I wanted the biggest one. __30__ I could say “ The biggest one”, one of my brothers stood up and said what I wanted to say.
My mum __31__ the biggest apple and said to us, “It is __32__to get the biggest apple. Many people want the best thing, but few really get it. Now let’s start a __33__. Look at the lawn(草坪) in front of our __34__. I’ll divide it into three parts. Each of you must finish pruning(修剪) __35__own share. The best and quickest one will be__36__the biggest apple.” I won the apple finally.
Since then, I have learned to try my best to succeed whenever I get a chance. I work __37__to get what I want. That was what my mother advised me to do. I feel very thankful to my mum because she taught me a very basic but the most __38__lesson. It is quite fair that we can get what we want __39__competing (竞争). My mother kept teaching us in this way and we followed her __40__ all the time.
I do believe that mother is the first teacher of children, who can teach them to be a No.1 pursuing person.
小题1:
A.howB.whatC.thatD.when
小题2:
A.wantedB.drewC.hidD.brought
小题3:
A.toldB.answeredC.repliedD.asked
小题4:
A.AllB.BothC.NeitherD.Either
小题5:
A.BeforeB.WhenC.UntilD.After
小题6:
A.ate upB.looked upC.picked upD.gave up
小题7:
A.goodB.strangeC.badD.polite
小题8:
A.lessonB.partyC.conversationD.competition
小题9:
A.schoolB.churchC.houseD.library
小题10:
A.myB.ourC.yourD.their
小题11:
A.sentB.givenC.shownD.shared
小题12:
A.hardB.carelesslyC.easilyD.comfortably
小题13:
A.expensiveB.importantC.difficultD.traditional
小题14:
A.onB.withoutC.aboutD.by
小题15:
A.adviceB.exampleC.decisionD.plan
单项选择题