问题 选择题

一质量为m的物体在水平恒力F的作用下沿水平面运动,在to时刻撤去F,其v-t图象如图所示.已知物体与水平面间的动摩擦因数为μ,则下列关于F的大小及其做的功W的大小关系式中,正确的是(  )

A.F=μmg

B.F=2μmg

C.W=μmgvoto

D.W=μmgvoto

答案

答案:D

题目分析:在加速运动过程中由牛顿第二定律可知,在撤去拉力只剩摩擦力后,由以上两个式子联立可知拉力F=3μmg,AB错;速度时间图像中图线所围成的面积表示位移的大小,由此可知拉力施加作用的过程中位移为,所以拉力做功W=μmgvoto,D对;

点评:明确加速过程和减速过程的受力情况,速度时间图像中图线的斜率表示加速度大小,图线的面积表示位移的大小,灵活应用动能定理选择合适的研究过程,本题难度中等

单项选择题
完形填空
I was sitting in the surgical waiting area at the hospital waiting for word of my wife's operation. The small room was filled with other families and friends who were 36 waiting to hear how their loved ones were doing.
There was a young boy sitting quietly in the__37__.He held a small box of__38__in one hand and a tablet( 写字板)in the other. Every so often he'd place the tablet on his lap and draw something. As the hours passed I began to see the room almost__39__.And there was a strange silence.
“Daddy,when is Mommy going home?”the young boy asked,breaking the__40__.
“Tomorrow,”his father replied. The boy picked up his crayons and__41__something on the tablet.
I _42__him as he moved his feet back and forth,looking up at the ceiling and then toward the sunshine-filled window.“When's__43__,Daddy?”he asked. His father__44__him over to sit by him. “Come,_45__here next to me,”he said with a gentle tone in his voice.
__46__his things,the little boy walked slowly to his father's side from the corner.
The man_47__out and held his son by his shoulders.
“There are three days in life. Yesterday,today and tomorrow. One is over and done,one we live in,the other we__48__.We can't live in yesterday,but if we are happy enough today,we will be even happier tomorrow,”he said in an attempt to__49__the boy.
The little boy looked at him for a moment and said,“If Mommy is going home tomorrow,I'll be__50__. So,I want it to be tomorrow already, Daddy.”
Now hanging on every word of this__51__,I leaned forward to hear his response. It was__52__.
“Son,the one great thing about today is once it__53__,it is already almost tomorrow.”
I thought about how__54__it was. Even in my adult life. I still live waiting for what tomorrow may__55__.Now I know that it is already “almost tomorrow.”
小题1:
A.eagerlyB.patientlyC.happilyD.curiously
小题2:
A.chairB.cornerC.distanceD.shadow
小题3:
A.giftsB.toysC.chalksD.crayons
小题4:
A.darkB.fullC.emptyD.clean
小题5:
A.promiseB.ruleC.recordD.silence
小题6:
A.wroteB.drewC.printedD.carved
小题7:
A.followedB.foundC.watchedD.caught
小题8:
A.todayB.yesterdayC.futureD.tomorrow
小题9:
A.knockedB.orderedC.calledD.pulled
小题10:
A.standB.sitC.lieD.sleep
小题11:
A.GatheringB.ForgettingC.SavingD.Choosing
小题12:
A.gotB.tookC.setD.reached
小题13:
A.spendB.wasteC.killD.expect
小题14:
A.frightenB.satisfyC.persuadeD.warn
小题15:
A.happierB.calmerC.smarterD.sadder
小题16:
A.storyB.argumentC.conversationD.discussion
小题17:
A.humorousB.sillyC.positiveD.perfect
小题18:
A.endsB.startsC.passesD.lasts
小题19:
A.meaningfulB.importantC.interestingD.fantastic
小题20:
A.happenB.comeC.bringD.need