问题 选择题

如图所示,质量为m1和m2的两个材料相同的物体用细线相连,在大小恒定的拉力F作用下做加速运动,先沿水平面,再沿斜面(斜面与水平面成 角),最后竖直向上运动,则在这三个阶段的运动中,细线上弹力的大小情况是(  )

A.由大变小

B.由小变大

C.始终不变

D.由大变小再变大

答案

设物体与接触面的动摩擦力因素为μ,在水平面有:

a1=

F-μ(m1+m2)g
m1+m2
=
F
m1+m2
g

对m1进行受力分析则有:

T1-μm1g=m1a1=

m1F
m1+m2
-μm1g

所以T1=

m1F
m1+m2

在斜面上有:

a2=

F-μ(m1+m2)gcosθ-(m1+m2)gsinθ
m1+m2

对m1进行受力分析则有:

T2-μm1gcosθ-mgsinθ=m1a2

解得T2=

m1F
m1+m2

竖直向上运动运动时有:

a3=

F-(m1+m2)g
m1+m2

对m1进行受力分析则有:

T3-m1g=m1a3

解得:T3=

m1F
m1+m2

所以绳子的拉力始终不变,故C正确

故选C

单项选择题
阅读理解

A man once said how useless it was to put advertisements in the newspapers. “Last week,” said he, “my umbrella was stolen from a London church. As it was a present, I spent twice its worth in advertising ,but didn’t get it back ”

“How did you write your advertisement ?”asked one of the listeners ,a merchant .

“Here it is,” said the man ,taking out of his pocket a slip cut from a newspaper. The other man took it and read, “Lost from the City Church last Sunday evening a black silk umbrella .The gentleman who finds it will receive ten shillings on leaving it at No 10 Broad Street .”

“Now ,”said the merchant, “I often advertise ,and find that it pays me well .But by the way in which an advertisement is expressed is of extreme importance .let us try your umbrella again ,and if it fails ,I will buy you a new one. ”

The merchant then took a slip of paper out of his pocket and wrote :“If the man who was seen to take an umbrella from the City Church last Sunday evening doesn’t wish to get into trouble, he will return the umbrella to No. 10 Broad Street .He is well known .”

This appeared in the paper ,and on the following morning ,the man was astonished when he opened the front door. In the doorway lay at least twelve umbrellas of all sizes and colors that had been thrown in and his own was among the number. Many of them had notes fastened to them saying that they had been taken by mistake, and begging the loser not to say anything about the matter

小题1:This is a story about ________.           

A.how a man lost and found his umbrella.

B.how to make an effective advertisement.

C.how to find lost things.

D.how to put an advertisement in the newspaper.小题2:“If it fails, I will buy you a new one ” suggested that _______.

A.he was rich enough to afford an umbrella

B.he was not sure he would get the umbrella back

C.he was quite sure of his success .

D.he was ready to help others .小题3:The result of the first advertisement was that ________.

A.the man got his umbrella back .

B.the man wasted some money advertising .

C.the man found his umbrella

D.someone found his umbrella .小题4:According to the first advertisement ,anyone who ___would receive ten shillings.

A.left the umbrella in the City Church

B.found the umbrella at No. 10 Broad Street

C.gave the message to the man

D.left the umbrella at No. 10 Broad Street