题干

如图所示,轻绳OA一端系于天花板上,与竖直方向的夹角为30°,水平轻绳OB的一端系于竖直墙上,O点挂一重物.如果绳OA能承受的最大拉力是300N,其余两绳能承受的拉力足够大,那么在O点最多能挂多重的重物?此时绳OB的拉力是多大?

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答案(点此获取答案解析)

解:根据O点受力由正交分解有:

FOAcos30°=G

FOAsin30°=FOB

由于FOA=300N

所以G=300× 32

同类题2

Choose the best answer and complete the passage(选择最恰当的选项完成短文)(12分)

    The following are the posts in an Internet chat room. In this chat room, parents talk about some of the problems they have with their children.

    It is ever OK to lie to a child?My 10-year-old son plays soccer.He's not very good,but he loves it.Yesterday he played very 1.When he finished the game,he asked,"Did I play well?"I said,"Yes!You are a great scoor player!"Did I do the right thing?

    Yes,you did the right thing.White lies are a(n)2part of life.Sometimes you have to tell lies that do no harm.Your purpose was to make him feel good.Now he's ready to play soccer again.

    I don't think there is ever a good reason to lie to our children.Children learn honesty from the3they see.As parents, we should be honest in order to teach honesty to our children.Every lie requires five more lies.If you keep hiding truth from them, they will not   4 you.They will not respect you.Honesty is the first step to a good parent-child relationship.

    You lied to make your son feel better, but you 5 a good chance to teach him a life lesson.People should work hard for what they want.Maybe next time you can say, "No,you didn't play well today.Great soccer players practise a lot. Let's go and practise."The truth will make him6and a good soccer player.

    Parents should be honest to their childern,but they don't have to tell the whole truth. Next time when your son asks, "Did I play well?"You can say,"What do you think?" Then your son can tell you what he thinks.That way,everything you say is true, and you avoid truth that hurts.