问题 问答题

(1)如图1,在光滑水平长直轨道上,放着一个静止的弹簧振子,它由一轻弹簧两端各连接一个小球构成,两小球质量相等.现突然给左端小球一个向右的速度u0,求弹簧第一次恢复到自然长度时,每个小球的速度.

(2)如图2,将N个这样的振子放在该轨道上,最左边的振子1被压缩至弹簧为某一长度后锁定,静止在适当位置上,这时它的弹性势能为E0.其余各振子间都有一定的距离,现解除对振子1的锁定,任其自由运动,当它第一次恢复到自然长度时,刚好与振子2碰撞,此后,继续发生一系列碰撞,每个振子被碰后刚好都是在弹簧第一次恢复到自然长度时与下一个振子相碰.求所有可能的碰撞都发生后,每个振子弹性势能的最大值.已知本题中两球发生碰撞时,速度交换,即一球碰后的速度等于另一球碰前的速度.

答案

(1)设每个小球质量为m,以u1、u2分别表示弹簧恢复到自然长度时左右两端小球的速度.

由动量守恒和能量守恒定律有

mu1+mu2=mu0(以向右为速度正方向)

1
2
m
u21
+
1
2
m
u22
=
1
2
m
u20

解得u1=u0,u2=0或u1=0,u2=u0

由于振子从初始状态到弹簧恢复到自然长度的过程中,弹簧一直是压缩状态,弹性力使左端小球持续减速,使右端小球持续加速,因此应该取u1=0,u2=u0

即弹簧第一次恢复到自然长度时,左侧小球速度为0,右侧小球速度为u0

(2)以v1、v1′分别表示振子1解除锁定后弹簧恢复到自然长度时左右两小球的速度,规定向右为速度的正方向,

由动量守恒和能量守恒定律,

mv1+mv1′=0

1
2
m
v21
+
1
2
mv
21
=E0

解得

v1=

E0
m
v1=-
E0
m
v1=-
E0
m
v1=
E0
m

在这一过程中,弹簧一直是压缩状态,弹性力使左端小球向左加速,右端小球向右加速,故应取v1=-

E0
m
v1=
E0
m

振子1与振子2碰撞后,由于交换速度,振子1右端小球速度变为0,左端小球速度仍为v1,此后两小球都向左运动,当它们向左的速度相同时,弹簧被拉伸至最长,弹性势能最大,设此速度为v10,根据动量守恒定律:

2mv10=mv1

用E1表示最大弹性势能,由能量守恒有

1
2
m
v210
+
1
2
m
v210
+E1=
1
2
m
v21

解得        

E1=

1
4
E0

振子2 被碰撞后瞬间,左端小球速度为

E0
m
,右端小球速度为0.以后弹簧被压缩,当弹簧再恢复到自然长度时,根据(1)题结果,左端小球速度v2=0,右端小球速度v2=
E0
m
,与振子3碰撞,由于交换速度,振子2右端小球速度变为0,振子2静止,弹簧为自然长度,弹性势能为E2=0.

同样分析可得

E2=E3=…EN-1=0

振子N被碰撞后瞬间,左端小球速度 vN-1=

E0
m
,右端小球速度为0,弹簧处于自然长度.此后两小球都向右运动,弹簧被压缩,当它们向右的速度相同时,弹簧被压缩至最短,弹性势能最大.此速度为vN0,根据动量守恒定律,

2mvN0=mv´N-1

用EN表示最大弹性势能,根据能量守恒,有

1
2
m
v2N0
+
1
2
m
v2N0
+EN=
1
2
m
v2N-1

解得       

 EN=

1
4
E0

故所有可能的碰撞都发生后第一个弹簧振子的最大弹性势能为

1
4
E0,第二个到第N-1个弹簧振子的最大弹性势能为0,第N个弹簧振子的最大弹性势能为
1
4
E0

阅读理解

I got my first driver's license(执照)in 1953 by taking driver education in my first year at Central High School in Charlotte,North Carolina. Four years later when it was time to renew my license I was a married woman. Henry and I were living in Baltimore,Maryland. Two weeks before my 20th birthday,Henry drove me to the motor vehicle office on a hot July afternoon. When I got to the office and showed to the man behind the counter my North Carolina driver’ s license,ready to renew,the man told me that I was under age by Maryland law since I was not yet 21.“Mr.Henry Smith,your husband,will have to sign for you,”he said.

I argued,pointing to a very large belly(肚子)of mine,“I am married. I am having a baby. Why should I have to have someone sign for me to drive?”He answered coldly,“It’s the law,madam.”

Henry encouraged me to calm down,just go ahead and get the license and be done with it. “No,” I said. I refused to have him sign for me. So I left without a Maryland license.

I called the North Carolina Motor Vehicle Office and renewed my NC license by mail—using my name Susan Brown. And thus it was for the next twelve years. Since Henry was in the army I could drive under my home state license. By the time Henry left the army we were once again living in Maryland,and I had to take the Maryland driver’ s exam. Since then I just go in and renew every four years—sign the name Susan Brown,have my new picture taken,and walk out with a license to drive.

小题1:Susan got her first driver’ s license________.

A.before she got married to Henry

B.when she was twenty years old

C.after she finished high school

D.when she just moved to Maryland小题2:Susan failed to renew her license the first time in Maryland because________.

A.she was forbidden to drive by Maryland law

B.she lacked driving experience in Maryland

C.she was to give birth to a baby soon

D.she insisted on signing for herself小题3:We can infer from the text that in the U.S.________.

A.American males should serve in the army

B.different states may have different laws

C.people have to renew their licenses in their home states

D.women should adopt their husbands’ family names after marriage

单项选择题