问题 多选题

如图所示,在粗糙水平面上放一三角形木块a,当b按下列四种不同方式运动时,a三角形物体始终对地静止,试问,在哪种或哪几种情形下,a三角形物体对地面有向右的静摩擦力.(  )

A.b物体沿斜面加速下滑

B.b物体沿斜面减速下滑

C.b物体沿斜面匀速下滑

D.b物体受到一次冲击后沿斜面减速上滑

答案

设a和b质量分别为M和m,斜面的倾角为θ,b的加速度大小为a.以a、b组成的整体为研究对象,分析受力情况,作出力图.由题,a三角形物体对地面有向右的静摩擦力,根据牛顿第三定律得知,地面对a有向左的摩擦力.根据牛顿第二定律得

   水平方向:f=max=macosθ

f方向向左,则ax方向也向左,b物体的加速度方向必然沿斜面向下,所以b物体沿斜面加速下滑或沿斜面向上减速运动.故AD正确.

故选AD

填空题
阅读理解

As is known to all, colors appear in every language to express people’s feeling and thoughts. Then, what is the situation in American English?

Red is a hot color. Americans may say they are red hot about something unfair. They are red hot when they are very angry about something. The small hot-tasting peppers found in many Mexican foods are called red hot for their color and their fiery taste. Fast loud music is popular with many people. They may say the music is red hot, especially the kind called Dixieland Jazz.

Pink is a lighter kind of red. People sometimes say they are in the pink when they are in good health. The expression was first used in America at the beginning of the twentieth century. It comes from the fact that many babies are born with nice pink skin that shows that they are in good health.

The color black is often used in expressions. People describe a day on which everything goes wrong as a black day. People or things on a blacklist are connected with things illegal now. But at one time, some businesses refused to employ people who were on a blacklist for belonging to unpopular organizations.

The color green is natural for trees and grass. But it is an unnatural color for humans. A person who has a sick feeling in his stomach may say he feels a little green. A passenger on a boat who is feeling very sick from high waves may look very green.

Sometimes a person may be upset because he does not have something as nice as a friend has. That person may say he is green with envy. Some people are green with envy because a friend has more dollars or greenbacks. Dollars are called greenbacks because green is the color of the back side of the paper money.

小题1:Americans use “red hot” to describe the following EXCEPT _______.

A.something unfair

B.small hot-tasting peppers

C.the person who is very angry

D.popular music like Dixieland Jazz小题2:When we say someone feels a little green, it means he/she _______.

A.enjoys himself in boating

B.is hit by a high wave

C.has a stomachache

D.likes trees and grass小题3:In the writer’s eyes, what is related to a black day?

A.Being sent a beautiful gift.

B.Passing a very difficult test.

C.Failing in an important interview.

D.Being invited to an exciting party.