问题 多项选择题

如图所示,一质量为m的木块靠在竖直粗糙墙壁上,且受到水平力F的作用,下列说法正确的是()

A.若木块静止,则木块受到的静摩擦力大小等于mg,方向竖直向上

B.若木块静止,当F增大时,木块受到的静摩擦力随之增大

C.木块沿墙壁向下运动时,墙壁对木块的弹力始终与力F等大反向

D.若木块静止,当撤去F,木块沿墙壁下滑时,木块不受滑动摩擦力作用

答案

参考答案:A, C, D

解析:试题分析:A、若木块静止,根据二力平衡,木块受到的静摩擦力大小等于mg,方向与重力方向相反;正确 B、静摩擦力与压力大小无关;错误C、墙壁对木块的弹力与水平力F是一对平衡力,等大反向;正确D、当撤去F,木块对墙壁没有了压力,也就没有了摩擦力;正确故选ACD考点:静摩擦力的大小点评:容易题。静摩擦力F的大小、方向都跟产生相对运动趋势的外力密切相关,但跟正压力无直接关系.因而F具有大小、方向的可变性,变化性强是它的特点.若为平衡状态,静摩擦力可由平衡条件建立方程求解.

阅读理解

The common cold is the world's most widespread illness, which is plagues(疫病) that flesh receives.

The most widespread mistake of all is that colds are caused by cold. They are not. They are caused by viruses(病毒) passing on from person to person. You catch a cold by coming into contact, directly or indirectly, with someone who already has one. If cold causes colds, it would be reasonable to expect the Eskimos to suffer from them forever. But they do not. And in isolated Arctic regions explorers have reported being free from colds until coming into contact again with infected people from the outside world by way of packages and mail dropped from airplanes.

During the First World War soldiers who spent long periods in the trenches(战壕), cold and wet, showed no increased tendency to catch colds.

In the Second World War prisoners at the notorious Auschwitz concentration camp(奥斯维辛集中营), naked and starving, were astonished to find that they seldom had colds.

At the Common Cold Research Unit in England, volunteers took part in Experiments in which they gave themselves to the discomforts of being cold and wet for long stretches of time. After taking hot baths, they put on bathing suits, allowed themselves to be with cold water, and then stood about dripping wet in drafty room. Some wore wet socks all day while others exercised in the rain until close to exhaustion. Not one of the volunteers came down with a cold unless a cold virus was actually dropped in his nose.

If, then, cold and wet have nothing to do with catching colds, why are they more frequent in the winter? Despite the most pains—taking research, no one has yet found the answer. One explanation offered by scientists is that people tend to stay together indoors more in cold weather than at other times, and this makes it easier for cold viruses to be passed on.

No one has yet found a cure for the cold. There are drugs and pain suppressors(止痛片) such as aspirin, but all they do is to relieve the symptoms(症状).

小题1:The writer offered _______ examples to support his argument.

A.4

B.5

C.6

D.3小题2:Which of the following does not agree with the chosen passage?

A.The Eskimos do not suffer from colds all the time.

B.Colds are not caused by cold.

C.People suffer from colds just because they like to stay indoors.

D.A person may catch a cold by touching someone who already had one.小题3:Arctic explorers may catch colds when ________.

A.they are working in the isolated Arctic regions

B.they are writing reports in terribly cold weather

C.they are free from work in the isolated Arctic regions

D.they are coming into touch again with the outside world小题4:The passage mainly discusses ________.

A.the experiments on the common cold

B.the fallacy about the common cold

C.the reason and the way people catch colds

D.the continued spread of common colds

单项选择题