问题 问答题 论述题

机油的黏度对发动机工作有哪些影响?

答案

参考答案:

黏度是液体流动时内摩擦力的量度。黏度可用绝对黏度和相对黏度表示。机油的黏度对发动机的工作有很大影响,黏度太小,不能在摩擦表面形成足够厚的油膜,是磨损加剧,密封性下降,汽缸漏气,使发动机功率下降;黏度太大,使发动机启动困难,油泵送性能差,容易出现干摩擦或半干摩擦,而且油的循环速度慢,冷却和洗涤作用差。

因此使用中要求机油的黏度要适中。

问答题
阅读理解

What will man be like in the future—in 5000 or even 50, 000 years from now? We can only make guesses, of course, but we can be sure that he will be different from what he is today, for man is slowly changing all the time.

Let us take an obvious example. Man, even five hundred years ago , was shorter than he is today. Now, on average,  men are about three inches taller. Five hundred years is a relatively short period of time, so we may assume that man will continue to grow taller. Again, in the modem world we use our brains a great deal. Even so, we still make use of only about 20% of the brain’ s capacity(容量). As time goes on , however , we shall have to use our brains more and more,  and eventually we shall need larger ones! This is likely to bring about a physical change too: the head, in particular the forehead, will grow larger. Nowadays our eyes are in constant use. In fact , we use them so much that very often they become weaker and we have to wear glasses. But over a very long period of time it is likely that man’ s eyes will grow stronger.

On the other hand, we tend to make less use of our arms and legs. These, as a result, are likely to grow weaker. At the same time, however , our fingers will grow more sensitive(敏感的) because they are used a great deal in modern life.

But what about hair? This will probably disappear from the body altogether in course of time because it does not serve a useful purpose any longer. In the future, then, both sexes are likely to be bald.

Perhaps all this gives the impression that future man will not be a very attractive creature to look at! This may well be true. All the same, in spite of all these changes, future man will still have a lot in common with us. He will still be a human being, with thoughts and emotions similar to our own. …

小题1:The passage mainly tells us that _______.

A.man’s life will be different in the future.

B.future man will look quite different from us.

C.man is growing taller and uglier as time passes.

D.human’s organs’ functions will become weak “小题2:The change in man’s size of forehead will probably be because ___________.

A.he makes use onIy20% of the brain’s capacity.

B.his brain has grown larger over the past centuries.

C.the other 80% of his brain will grow in due time

D.he will use his brain more and more as time goes on :小题3:What will be true about a human being in the future?

A.He will be hairless because hair is no longer useful.

B.He will have smaller eyes and will wear better glasses.

C.His fingers will grow weaker because he won’t have to make use of them.

D.He will think and feel in a different way.小题4:It is implied that ________.

A.human beings will become less attractive in the future

B.less use of a bodily organ may lead to its degeneration(退化,)

C.human beings hope for a change in the future life

D.future life is always predictable