问题 问答题

图(a)是杂技芭蕾天鹅舞中的一个动作,为了估算扮演王子的男演员右手需用拉力的大小,如图(b)所示,可将被托举的白天鹅女演员简单地抽象为一个可绕通过O点且垂直于纸面的轴转动的物体AOB,其中OB与水平方向之间的夹角为θ=45°,设AO段的质量为m1=36kg,其重心C与O点的距离为CO=0.4m;女演员的腿部长OB=0.7m,质量为m2=8kg,且均匀分布,试计算水平拉力F的大小.

答案

受力分析如图所示

以O为轴:M=M

m1g×OC=F×OB×sinθ+m2

OB
2
OC×cosθ

36×10×0.4=F×0.7×sin45°+8×10×0.35×cos45°

F≈250.92N

答:水平拉力F的大小为250.92N.

单项选择题
阅读理解

When asked to point out one or two things that are most important to themselves, many put friends ahead of homes, jobs, cloth and cars.

A true friendship carries-a-long history of experience that determines who we are and keeps us connected. It is a treasure we should protect. Unfortunately, the better friends you are, the more probably you’ll have disagreements. And the result can be what you don’t want an end to the relationship.

The good news is that most troubled friendships can be mended First, don’t let your pride get in your way. Most of us can forgive each other when differences are brought out in the open . second, apologize when you’re wrong – even if you’ve been wronged. Over the course of a friendship, even the best people make mistakes. Sometimes, it may be best if the wronged person takes the lead and apologizes. When you apologize, give your friend a chance to admit that he has been wrong. Third, see things from your friend’s point of view. And finally, accept that friendships change as our needs and lifestyle change. Making friends can sometimes seem easy. The hard part is keeping the connections strong during the natural ups and downs that have an effect on all relationships . My suggestion: Consider friendship an honor and a gift , and worth the effort to treasure and nurture .

小题1:What would be the best title for the text?

A.Easy Ways to Make Friends

B.Ups and Downs in Friendship

C.How to Mend a Troubled Friendship

D.How to Take the Lead in Making Friends小题2:The “wronged person” underlined in the text refers to a person           .

A.who has been mistaken for another

B.who has been blamed unfairly

C.who has treated friends badly

D.who has admitted his mistakes小题3:According to the text a friendship can last long only if             

A.we have much in common

B.we know our friends’ mistakes

C.we treat our disagreements wisely

D.we have know one another for long小题4:What should we do if we follow the author’s second suggestion?

A.Stick to our own prints of view

B.Avoid making mistakes

C.Make an apology first

D.Change our lifestyles