问题 填空题

用一只动滑轮匀速提起800N的重物.若动滑轮重200N,不计绳重及摩擦,所用的拉力F=______N;如果用它提起更重的物体,则它的机械效率将______(增大/减小/不变).

答案

(1)使用一只动滑轮时,若不计绳重及摩擦,则:

提起这个重物的拉力F=

1
2
(G+G)=
1
2
×(800N+200N)=500N.

(2)如果用它提起更重的物体,根据W有用=Gh可知有用功会增加,

而在不计绳重及摩擦的情况下,产生的额外功为W额外=Gh,保持不变,

所以总功在增加,所以根据机械效率公式η=

W有用
W
=1-
W额外
W
可知:

它的机械效率将增大.

故答案:500;增大.

阅读理解

The benefits of keeping a journal (日志)

If you want to grow, one important thing you should do is keeping a journal. It may seem simple, but it can make a big difference in your life. I have been keeping journals for years. Writing all the lessons I learn and all the ideas I get has become a habit for me.    小题1:   Here are some benefits you will get by keeping a journal.

◆ It prevents you from losing an idea. Have you ever gotten an idea only to lose it later because you didn’t write it down?   小题2:   But then I developed the habit of writing down every idea that came into my mind as soon as possible. If I’m away from my computer, I usually write it down on a piece of paper that I bring wherever I go. I will then transfer the idea to the journal in my computer.

◆ It helps you review all the lessons you’ve learned. By reviewing your journal, you can quickly see the lessons you’ve learned and the ideas you’ve gotten.    小题3:   You can use the ideas to propel (鞭策) yourself forward.

   小题4:   After keeping a journal for years, you can look back at it and see how far you’ve gone. Things that were big problems in the past might seem small today. The raw ideas you had in the past might have been realized today.

◆ It helps you expand your ideas. When you try to come up with a sentence to express an idea, you are thinking actively about it.    小题5:   In the end, you will expand your ideas.

A.I often experienced that myself.

B.It trains you to express your thoughts.

C.It allows you to see your progress over time.

D.Just use whatever tool you feel comfortable with.E. Thinking actively helps you connect your idea to another idea.

F. And to be honest, I can’t imagine what my life would be like without it.

G. Then you can do whatever necessary to avoid repeating the same mistakes.

单项选择题 A1型题