问题 问答题 简答题

动滑轮为何比定滑轮省一半的力?

答案

参考答案:

定滑轮轴位置固定不变钢丝绳绕过滑轮,能改变起升重物时力的方向,它相当于一个等臂杠杆,不省力只改变力的方向。动滑轮轴位置不固定,随重物的移动而移动,它不但改变力的方向,而且相当于一个动力臂为阻力臂2倍的杠杆原理因此提起重物时节省一半的力。

阅读理解

James is a good student and he has lots of friends,but he also has a problem.Some older boys are bullying(欺负)him at school.James is very unhappy and he doesn't know what to do about it.Here are some suggestions to him and other teenagers in this situation.

Don't feel worried.It's not your fault(过错)! Being bullied can make you feel very lonely and angry,but you are not alone.Don't feel that you have to hide the problem.You should find a person you can trust,and tell them.It might be your teacher,your parents,or even your friend's parents.After you tell someone,you will get some support and feel some relief(解脱).

Speaking to an adult might make you nervous,but here are other things you can do.Some people express their feelings more easily on paper.Write a letter to someone or keep a diary.Include all the details(细节)about what the bullies do, as well as when and where the bullying happens.You can use it as proof to show what is going on.And it is a wonderful idea to show your letter or diary to a teacher or another responsible adult.Then the bullies will feel very afraid if their names appear in a letter!

Also,don't show you are sad and don't try and fight with the bullies. You could get in trouble yourself. Ignore them and just walk away. The bullies will soon stop.

小题1:What do you think the text is trying to tell us?

A.What to do about being bullied.

B.How to be a good student at school.

C.What to write well in a diary.

D.How to give suggestions to teenagers.小题2:You'd better go and find a person you can trust and ____.

A.know what happens

B.learn from them

C.ask for help

D.make them lonely小题3: If you remember and follow the suggestions above, ____.

A.nobody will speak to the bullies

B.everybody will feel some relief

C.someone will express their feelings

D.the bullies will soon stop小题4:What does the underlined word "ignore" mean in the text?

A.take no notice of

B.take care of

C.make full use of

D.get more help from

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