问题 问答题

已知斜面长12米,高2.4米,效率80%.若用10秒钟将103牛顿重的物体匀速沿斜面方向由斜面底端拉至顶端,试求:

①拉力?②有用功?③额外功?④摩擦力?⑤人拉物体的功率?

答案

(1)物体从斜面的底端拉到顶端时,所做的有用功:

W有用=Gh=103N×2.4m=247.2J;

(2)∵机械效率η=

W有用
W

∴W=

W有用
η
=
247.2J
80%
=309J,

∵W=FL,L=12m,

∴沿斜面向上的拉力:

F=

W
L
=
309J
12m
=25.75N;

人拉物体的功率:

P=

W
t
=
309J
10s
=30.9W;

(3)∵W=W有用+W额外

∴W额外=W-W有用=309-247.2J=61.8J

∵W额外=fL,

∴摩擦力:

f=

W额外
L
=
61.8J
12m
=5.15N;

答:①拉力为25.75N;②有用功为247.2J;③额外功为61.8J;④摩擦力为5.15N;⑤人拉物体的功率为30.9W.

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

We all have the ability to make friends. 46 Making friends is a skill, and the first step in developing that skill is knowing how to get along with others. People who are good at getting along with others take an interest in what other people like.

The following are some tips.

²    Realize your own strengths. 47 Some people don’t let others see their goodness.

²    Believe that you are a good person who has something to share with others.

²    Develop a sense of humor. Learn to laugh at yourself and your shortcomings. It will give out some of the stress in your life.

²    48 They have their opinions, and you have yours. Different opinions are healthy. Learn how to listen to others without putting them down or trying to persuade them that your opinion is right.

²    Be kind. Throughout your life, you will learn that if you are kind to others, they will usually be kind to you. 49 Right now. Give someone a compliment(恭维). Open the door for someone. Wave to a friend. Encourage someone who is discouraged. There are hundreds of little things you can do to show kindness.

²    Be sympathetic. 50 Look at life through their eyes.

²    Never give up. Even if you are faced with negative, closed-minded people, never give up. Don’t let them change you into one of them. Find people who will appreciate you.

A.Respect the rights of others.

B.When should you start being kind?

C.Unfortunately, many people never reach their potential.

D.Friends are very important to every one of us.E.       You have skills and talents that others can enjoy.

F.       You must try your best to help others out of trouble.

G.       Try to understand what others are going through.

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