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（选修模块3-4）

利用单摆测量某地的重力加速度，现测得摆球质量为m，摆长为L，通过传感器测出摆球运动时位移随时间变化的规律为x=Asin（ωt+φ），则该单摆的振动周期为______，该处的重力加速度g=______．

答案

由摆球运动时位移随时间变化的规律为x=Asin（ωt+φ），知摆球的圆频率为ω，则单摆的周期T=

2π
ω
．

根据T=2π

L
g
解得，g=
4π2L
T2
=
4π2L
4π2
ω2
=ω2L．

故答案为：

2π
ω
，ω2L

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单项选择题

According to the United States government, people are classified as homeless if they have no place to stay and no expectation of finding a place for the next thirty days, Although technically accurate, that is an impersonal assessment of an enormous and very human problem,
The homeless population represents all of us Americans, It includes men and women, the elderly, children, and infants. Its members are from all ethnic (种族的) groups. What they have in common is poverty.
Currently in the U.S., thirty-nine million people live in poverty. When money is really tight, paying the rent or buying food often becomes a choice. Government assistance in the form of food stamps does help but, as one homeless man explains, you can’t pay the rent with food stamps.
With no money for rent, the streets and homeless shelters become the alternative.

Most of the homeless in the U.S. are ______.
A. male adultsB. female adults C. the elderlyD. children

Although men constitute the largest group within the homeless population, homeless women with children are rapidly joining them. In fact, one quarter of the homeless people in the U.S. are teenagers and young children.
People may become homeless for numerous reasons. However there are certain factors that many of these individuals have in common. They include a lack of adequate education and job skills. A majority of the teenagers and adults have not completed high school.
The abuse of alcohol and drugs is also a common factor. One third of the adult homeless population abuses alcohol, while one quarter of the same group uses drugs.
Some members of this population suffer mental health problems. Within the past several years many institutions for the menially ill have been closed and their patients sent "home". Unfortunately, a number of those people have no home to go to and they are unable to adequately look after themselves.
Job loss in today’s economy has also become a real factor in the loss of people’s homes. The breakup of families through abandonment and divorce are also contributing factors, particularly when there are children involved. The parent who is left to care for the kids with inadequate income may be forced to depend on the homeless shelters to put a roof over their heads.
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