问题 选择题

质量分别为M和m的两物体靠在一起放在光滑水平面上.用水平推力F向右推M,两物体向右加速运动时,M、m间的作用力为N1;用水平力F向左推m,使M、m一起加速向左运动时,M、m间的作用力为N2,如图甲、乙所示,则(  )

A.N1:N2=1:1

B.Nl:N2=m:M

C.N1:N2=M:m

D.无法比较N1、N2的大小

答案

用水平推力F向右推M时,

对Mm整体由 F=(M+m)a1

对于m由 N1=ma1

所以 N1=

mF
M+m

用水平力F向左推m时,

对Mm整体由 F=(M+m)a2

对于M由 N2=Ma2

所以N2=

MF
M+m

所以Nl:N2=m:M,所以B正确.

故选B.

阅读理解

How should one invest a sum of money in these clays of inflation (通货膨胀)? Left in a bank it will hardly keep its value, however high the interest rate. Only a brave man, or a very rich one, dares to buy and sell on the Stock Market. Today it seems that one of the best ways to protect your savings, and even increase your wealth is to buy beautiful objects from the past. Here I am going to offer some advice on collecting antique clocks, which I personally consider are among the most interesting of antiques.

I sometimes wonder what a being from another planet might report back about our way of life. "The planet Earth is ruled by a mysterious creature that sits or stands in a room and makes a strange ticking sound. It has a face with twelve black marks and two hands. Men can do nothing without its permission, and it fastens its young round people's wrists so that everywhere men go they are still under its control. This creature is the real master of Earth and men are its slaves(奴隶)."

Whether or not we are slaves of time today depends on our culture and personality, but it is believed that many years ago kings kept special slaves to tell the time. Certain men were very clever at measuring(量) the time of clay according to the beating of their own hearts. They were made to stand in a fixed place and every hour or so would shout tire time. So it seems that the first clocks were human beings.

However, men quickly found more convenient and reliable(可靠的) ways of telling the time. They learned to use the shadows cast by the sun. They marked the hours on candles, used sand in hour- glasses, and invented water-clocks. Indeed, any serious student of antique should spend as much time as possible visiting palaces, stately homes and museums to see some of the finest examples of clocks from the past.

Antique clocks could be very expensive, but one of the joys of collecting clocks is that it is still possible to find quite cheap ones for your own home. After all, if you are going to be ruled by time, why not invest in air antique clock and perhaps make a future profit(收益)?

小题1:According to the passage, collecting antique clocks____.

A.can hardly keep the value of your savings

B.will cost much of your savings

C.may increase your wealth

D.needs your bravery小题2:By quoting (引用) the remark of a being from another planet, the author intends to____.

A.suggest human beings are controlled by a clock

B.describe why clocks can rule the planet Earth

C.tell readers what clocks look like

D.compare chocks to human beings小题3:Which of the following is NOT mentioned as a way to measure the time?

A.Counting the beating of one's own heart.

B.Making use of candles, sand and water.

C.Observing shadows cast by the sun.

D.Keeping slaves busy day and night.小题4:The underlined phrase stately homes in paragraph(段落)4 means_________.

A.state-owned houses

B.houses in very good condition

C.grand houses open to the public

D.houses where statesmen meet regularly小题5:The purpose of the passage is________.

A.to introduce the culture of antique clocks

B.to offer some advice on collecting antique clocks

C.to compare different ways to make a future profit

D.to explain convenient and reliable ways of telling time

选择题