问题 问答题

如图的甲所示,A、B两点相距x=7m,一个质量m=2.0kg的木块开始静止在光滑水平面上的A点,木块受到如图乙的周期性水平力F的作用,规定向左方向为正方向.求:

(1)若力F的变化周期T=4s,则在t=0时刻将木块从A点无初速释放,木块到达B点时动能的大小;

(2)所加力F的频率f多大时,在t=

T
4
到t=
T
2
时间内从A点无初速释放该木块,木块不能到达B点.

答案

(1)木块所受力F=ma,根据牛顿第二定律,有:a=

F
m
=2m/s2

木块在0~

T
2
时间内向B点作匀加速直线运动,走过的距离为:x1=
1
2
a(
T
2
)2=4m

则木块此时的速度:v1=at2=4m/s

木块在t=

T
2
~T时间内继续向B点作匀减速直线运动可以到达B点;

位移为:x2=x-x1=3m

由公式:

v22
-
v21
=-2ax

所以,木块到达B点的速度:v2=2m/s

木块到达B点的动能:Ek=

1
2
mv2=4J

(2)木块在t=

T
4
~t=
T
2
向B点做匀加速运动,在t=
T
2
~t=
3T
4
向A点做匀减速运动,速度减为零后将返回.

木块向B点运动可能的最大位移:s=2×

1
2
a(
T
4
)2=
1
16
aT2

要求粒子不能到达A板,有:s<x=7m;

由f=

1
T
,力F变化频率应满足:f>
a
16x
=
1
28
14
Hz

答:(1)若力F的变化周期T=4s,则在t=0时刻将木块从A点无初速释放,木块到达B点时动能的大小为4J;

(2)所加力F的频率f>

a
16x
=
1
28
14
Hz时,在t=
T
4
到t=
T
2
时间内从A点无初速释放该木块,木块不能到达B点.

选择题
阅读理解

D

11-year-old Courtney Thompson was so passionate about gymnastics that she would practice five hours a day until her arms were sore and her elbows swollen.She rarely complained about her tough schedule,and for good reason.Courtney was ranked the best gymnast in the State for her age,and she dreamed of going to the Olympics.But on January 12th,2005,while practicing a routine exercise,she felt an unbearable pain in the left elbow.The medical examination then revealed a severe injury,resulting in an immediate surgery and a slow,painful recovery.

What happened to Courtney Thompson is a red flag in America's growing epidemic of youth sports injuries.Across the country,younger and younger athletes are injuring themselves in the pursuit of sports achievement.The U.S.Consumer Product Safety Commission reports that 2.8 million Americans aged 5 through 24 received medical treatment for a sports-related or recreation-related injury in 2003.Severe injuries that used to be limited to professionals are now showing up in high school and even junior high athletes.

There are many reasons behind the outbreak in sports injuries,hut most experts agree on one:the extreme culture of organized youth sports.Gone are the days when children played a variety of games on playgrounds.Child kidnapping and street crime have ended casual neighborhood athletics in many places,leading parents to enroll their children in organized sports under the watchful, though demanding,eyes of adult volunteers and coaches.And movies like Friday Night Lights glorify the thrill of victory for young athletes with single-minded dedication to athletic achievement.

From high school gyms to county hall fields,41 million children younger than 19 participated in organized youth sports in 2005,according to the latest research by the National Council of Youth Sports. The figure represents a 25 percent increase since 1997.

Parental involvement 1”youth sports has also made childhood games less about having fun and more about training and competing for the top spot.1anding a college scholarship or launching a promising career Hoping to nurture the next Michelle Kwan or Apolo Ohno,some parents guide their children into specializing in one sport But the repetitive training required to master a sport places enormous stresses on hones and muscles that are still developing.

67.The passage is mainly about_________

A.children’s participation in organized sports

B.the main causes of the youth sports injuries

C.the way to train children in organized sports

D.the common occurrence of youth sports injuries

68.The underlined part “Fed flag” in the second paragraph most probably means“_______”.

A.warning              B.problem      C. symbol       D.mark

69.Why is the number of sports injuries among children in America growing?

A.Because of their increasing fondness for sports.

B.Because children are not professional enough fur sports

C.Because more children are seeking for sports achievements.

D.Because of the involvement of the parents and more organized sports

70. We can learn from the passage that __________     

A.about 10.25 million youngsters participated in organized sports in 1997

B.parents give limited chances for children to choose their sports

C.serious injuries often happened to young students in the past

D.courtney Thompson will never become a famous gymnast