问题 计算题

(10分)如图所示,倾角θ=37°的斜面底端光滑的水平面BC平滑连接着半径r=0.40m的竖直光滑圆轨道。质量m=0.50kg的小物块,从距地面h=2.7m处沿斜面由静止开始下滑,小物块与斜面间的动摩擦因数μ=0.25,求:(sin37°=0.6,cos37°=0.8,g=10m/s2

(1)物块滑到斜面底端B时的速度大小。

(2)物块沿圆轨道运动到最高点A后在空中做平抛运动落在水平面BC上,已知平抛

运动水平位移为1.8m,求物体运动至A点时对圆轨道的压力大小。

答案

(1)m/s

(2)NA=20.3N

(1)物块沿斜面下滑过程中,在重力、支持力和摩擦力作用下做匀加速运动,

设下滑加速度为a ,到达斜面底端B时的速度为v,则:

   (2分)

            (2分)

代入数据解上述两式得:m/s       (1分)

(2)设物块运动到圆轨道的最高点A时的速度为vA,在A点受到圆轨道的压力为N,

则有:                       (2分)

由平抛运动规律得:                    (1分)

x=vAt             (1分)

解上述三式得:    N="20.3N             " (0.5分)

由牛顿第三定律可知:物块运动到圆轨道的最高点A时,对圆轨道的压力大小

NA="N=20.3N "       (0.5分)

阅读理解

The common Chinese greeting of “Ni chi le ma?” may soon be replaced by a new greeting: “Have you cleaned your plate?”

Over the Spring Festival holiday there was a big effort to get people to curb their habit of ordering too much food in restaurants because a lot of that food ended up being wasted. Holiday eaters were urged to “clean their plates” and were asked to take leftover food home in “doggy bags.”

The impetus behind the campaign was the startling government statistic that over 200 billion yuan is spent every year on food that ends up in the garbage bin. And in the midst of all this waste, statistics show that 128 million people are living below the poverty line and going hungry.

The “clean your plate” campaign is part of a worldwide effort to stop precious food from being wasted. The United Nations estimates that $1trillion (yes, that’s a very big number) worth of food is wasted each year. Most of that is lost during food production and transportation, but a significant percentage is attributed to waste by consumers. The United Nations Environment Program (UNEP) has launched a campaign called “Think. Eat. Save.” to make people aware of the problem.

Chinese actress Fan Bingbing is an ambassador for UNEP and she encouraged people to upload pictures of their “clean plates” to show their support for the program. Thousands of people responded and demonstrated that they had a clear conscience to go along with their full stomachs.

Xi Jinping, the incoming Chinese president, is also urging people to be less wasteful and he told Party members to cut back on extravagant holiday banquets and to be more careful when spending the people’s money.

For some, though, Xi Jinping’s and Fan Bingbing’s admonitions don’t go far enough. Yuan Longping, an agricultural scientist, caused a stir by saying that wasting food should be treated as a crime. (Jail time for leftover chicken feet!?)

If not a crime, wasting food is certainly a sin and it goes against what Xi Jinping calls, “the Chinese tradition of being diligent and thrifty.” Having a “clean plate” is just a traditional and responsible thing to do.

小题1:Which statement is NOT TRUE according to the passage?

A.Over 200 billion yuan is wasted every year on food around China.

B.It’s quite common for Chinese people to order more food than they need in restaurants.

C.It’s universally acknowledged that wasting food is a crime.

D.Fan did something to promote people’s awareness of saving food.小题2:What’ the best title for the passage?

A.Food is important.

B.Have you cleaned your plate?

C.Jail time for leftover chicken feet!?

D.Wasting food is crime.

单项选择题