问题 多选题

如图所示,三角体由两种材料拼接而成,BC界面平行于底面DE,两侧面与水平面夹角分别为30°和60°;已知物块从顶端A由静止下滑,先加速至B再匀速至D;若该物块由静止从顶端A沿另一侧面下滑,则(  )

A.物块一直加速运动到E,但AC段的加速度比CE段小

B.物块在AB段的运动时间大于AC段的运动时间

C.物块将先加速至C再匀速至E

D.物块通过C点的速率大于通过B点的速率

答案

AC、将物块在CE段与BD段运动进行比较可知,BD段上重力沿斜面向下的分力与滑动摩擦力大小相等.在CE段上重力沿斜面的分力较大,滑动摩擦力较小,则重力沿斜面的分力大于滑动摩擦力.所以物块沿CE段将做匀加速运动.由上分析可知,物块一直加速运动到E,由于AC段滑动摩擦力较小,则AC段的加速度比CE段大.故A、C错误.

BD、设△ABC的高度为h,物块滑到到B和C的速度大小分别为vB,vC,动摩擦因数为μ,斜面的倾角为α.根据动能定理得,对AB过程mgh-μmgcos30°

h
sin30°
=
1
2
mvB2,对AC过程,mgh-μmgcos60°
h
sin60°
=
1
2
mvc2
.由于μmgcos30°
h
sin30°
>μmgcos60°
h
sin60°
,则得到vB<vC.由位移公式得 xAB=
vB
2
tAB
xAC=
vc
2
tAC
,因为xAB>xAC,vB<vC,则tAB>tAC.故B、D正确.

故选BD.

完形填空

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.选项中有两项为多余选项

It seems that the Englishmen just cannot live without sports of some kind. 小题1:   

Wherever you go in this country, you will see both children and grown-ups knocking a ball about with a stick or something, as if in Britain men shall always remain boys and women girls! Still, it can never be bad to get exercise, can it?

Taking all amateur(业余)and professional sports in Britain into consideration, there can be no doubt that football is at the top of the list. 小题2: The game originated(起源于)in Britain and was played in the Middle Ages or even earlier, though as an organized game, or “association football”, it dates only from the beginning of the 19th century.

 小题3:  . It is a kind of football played by two teams of fifteen players than eleven. In rugby, an oval-shaped ball is used which can be handled as well as kicked. It is a pretty rough game.

In summer, cricket is the most popular sport. In fact, it has sometimes been called the English national game. Most foreigners find the game rather slow or even boring, but it enjoys great popularity among the British.

  小题4:  . It was introduced into England from France in the 15th century, but it was from England that it spread to practically every country in the world.

Table tennis, or “ping-pong, surely is not played on a great scale as it is in China or Japan.   

小题5:  . Horse-back riding, swimming, rowing and golf all attract a lot of people.

A.A famous French humorist once said that this is because the English insist on behaving like children all their lives.

B.It is called soccer in the United States

C.The next is rugby, which is called “football” in the United States

D.Tennis rates high on the list, tooE. Basketball and volleyball were introduced into Britain during the late 19th century from America and are gaining popularity

F. people often work out to build up their bodies.

G. not everyone likes ball games  

单项选择题