问题 选择题

从空中某处平抛一个物体,不计空气阻力,物体落地时,末速度与水平方向的夹角为。取地面物体重力势能为零,则物体抛出时,其重力势能与动能之比为

A.

B.

C.

D.

答案

答案:C

题目分析:物体做平抛运动,假设落地速度为v,由于落地的速度方向与水平方向的夹角为α,故

水平分速度为

竖直分速度为

由于平抛运动的水平分运动为匀速直线运动,故

由于平抛运动的竖直分运动为自由落体运动,故高度为

抛出时的动能为

抛出时的势能为

因而势能与动能之比为

故选C.

点评:本题关键根据末速度的大小和方向,求解出抛出时的动能和势能的表达式,再求得比值.

判断题
阅读理解

阅读理解。

     请认真阅读下列短文,并根据所读内容在文章后表格中的空格里填入最恰当的单词。

     (注意:每空格1个单词)

     WASHINGTON -According to statistics compiled by the Federal Bureau of Investigation,

72 officers were killed by criminals in 2011, increased markedly in recently years.

     The 2011 deaths were the first time that more officers were killed by suspects than car accidents.

The number was the highest in nearly two decades, excluding those who died in the Sept. 11 attacks

in 2001 and the Oklahoma City bombing in 1995.

     While the F.B.I. and other law enforcement(执行) officials cannot fully explain the reasons for the

rise in officer homicides, they are clear about the terrible consequences.

     "In this law enforcement job, when you pin this badge on and go out on calls, when you leave

home, you can't guarantee that you will come back," said Sheriff Ray Foster of Buchanan County,

Va.

     After a series of killings in early 2011, Attorney General Eric H. Holder Jr. asked federal authorities

to work with local police departments to try to come up with solutions to the problem.

      The F.B.I., which has tracked officer deaths since 1937, paid for a study conducted by John Jay

College that found that in many cases the officers were trying to arrest or stop a suspect who had

previously been arrested for a violent crime.

     That prompted the F.B.I. to change what information it will provide to local police departments,

the officials said. Starting this year, when police officers stop a car and call its license plate into the

F.B.I.'s database, they will be told whether the owner of the vehicle has a violent history. Through the

first three months of this year, the number of police fatalities has dropped, though it is unclear why.

     Some law enforcement officials believe that techniques pioneered by the New York Police

Department over the past two decades and adopted by other departments may have put officers at

greater risk by encouraging them to conduct more street stops and to seek out and confront (对抗)

suspects who seem likely to be armed. In New York and elsewhere, police officials moved more

officers into crime-ridden areas.

     Some argue that the rise in violence is linked to the tough economy. With less money, police

departments, after years of staffing increases, have been forced to make cutbacks(削减).

     The police chief in Camden, N.J., J. Scott Thomson, whose force of 400 was cut by nearly half

last year because of financing issues, said that having fewer officers on the street "makes it that much

more difficult to create an environment in which criminals do not feel as encouraged to attack anothe

r person, let alone a law enforcement officer."

     "Every stop can be potentially fatal, so we are trying to make sure the officers are ready and

prepared to face deadly force every single day they go out." Ms. Klimt said.