问题 计算题

(15分)静止在水平地面上的木箱,质量m=50 kg.木箱与地面间的动摩擦因数μ=0.4,若用大小为400 N、方向与水平方向成37°角的斜向上的拉力拉木箱从静止开始运动,使木箱能够到达50 m远处,拉力最少做多少功?(cos37°=0.8,取g=10 m/s2)

答案

7.68×103 J

欲使拉力做功最少,须使拉力作用的位移最小,故重物应先在拉力作用下加速,再撤去拉力使木箱减速,到达50 m处时速度恰好减为0.设加速时加速度的大小为a1,减速时加速度的大小为a2.

由牛顿第二定律得,加速时有:

水平方向Fcos37°-μFN=ma1

竖直方向Fsin37°+FN-mg=0

减速时有:μmg=ma2

且有v2=2a1x1=2a2x2 x1+x2=x

联立以上各式解得:x1≈24 m

由功的定义,有

W=Fx1cos37°=400×24×0.8 J=7.68×103 J.

阅读理解

根据短文内容,从短文后的七个选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

Should Good Sportsmanship Be Taught in School?

Almost everyone feels that it is important for young people to win fairly and show respect toward others.Should good sportsmanship, then, be taught in school? Critics do not think so.     1    Presented here are the most important benefits of teaching sportsmanship.

Teaching good sportsmanship and having a positive attitude in activities like sports, class elections, and science fair competitions are critical to success in school.    2     Acting calmly under pressure helps young adults succeed not only in sports but also in life.

     3    A person cannot play by the rules if he or she does not understand what the rules are.Knowing the rules also applies to other activities.If students enter a science fair competition, for example, they must follow the guidelines. Those who know the rules are much more likely to succeed.When students learn sportsmanship, they discover, how knowing the rules relates to achievement.

Good players become skilled at handling responsibility.When people accept responsibility, they achieve their best.Positive thinkers, moreover, influence others, especially those who are struggling with a problem.A gifted basketball player, for example, might teach skills to a less-talented player.Helping others raises the performance of the entire team.The team, consequently, could reach new heights and set new records.     4    Those who do not practice sportsmanship pass off responsibility.For them, if an opportunity is lost, it is someone else' s fault.When students participate on a team, they benefit by learning to accept responsibility, encourage others and own up to(认错) mistakes.

Sportsmanship is a winning idea.It must be taught in school.Students need direction to become good sports".Students will be responsible for their own actions and will have a positive influence on others.As students enter the workforce, businesses benefit.The community also benefits because these students will someday become leaders.     5   

A.We need to educate students about how sportsmanship pays off for everyone.

B.However, the stronger argument favors teaching good sportsmanship.

C.Sports help young adults learn how to team up with others.

D.Good sportsmanship also means admitting mistakes.E.Activities involving winning or losing can be tense.

F.It is natural that players should be eager to win.

G.Good team players know the rules of the game.

单项选择题