问题 选择题

用绳系一个小球,使它在光滑水平桌面上绕O点做匀速圆周运动.小球运动到A点时绳突然断裂,则此后小球将

A.沿轨迹1运动

B.沿轨迹2运动

C.沿轨迹3运动

D.沿轨迹4运动

答案

答案:A

题目分析:根据牛顿第一定律可知:当物体不受力时,将保持匀速直线运动状态或静止状态;光滑的水平面没有摩擦,当绳子突然断裂,在水平方向上,不受拉力和摩擦力;在竖直方向上,受到的重力和支持力是一对平衡力,大小相互抵消;故小球相当于不受力,将保持绳子断裂时的速度做匀速直线运动.当绳子突然断裂,在水平方向上,不受拉力和摩擦力;在竖直方向上,受到的重力和支持力是一对平衡力,大小相互抵消;故小球相当于不受力,将保持绳子断裂时的速度做匀速直线运动.故A正确.

故选A.

点评:物体将处与什么运动状态,由物体的受力情况来决定.此题中当物体受平衡力作用时,将保持绳子断时的速度做匀速直线运动.

单项选择题 A1/A2型题
阅读理解

It is "one of the few bright spots in the Chinese economy," says Zeng Ming. He is talking about e-commerce. Mr Zeng, the chief strategy officer for Alibaba, a giant Chinese Internet firm, predicts that digital transactions on his firm's platforms will top l trillion yuan ($159 billion) this year-more than Amazon's and eBay's combined. That is a bold claim, but consider what happened on Singles Sunday.

Invented a few years ago by students and seized upon by digital marketers, this festival for lonely hearts falls annually on the  llth day of the llth month (since l is the loneliest number).It is like St Valentine's Day, only worse. Singletons shower each other with tender gifts: a barrage of pearls; a storm of sweets.

This November llth they spent a surprising 19 billion yuan on Alibaba's online platforms-a fourfold increase on a year ago, and more than double what Americans spent online last Cyber Monday (the Monday after Thanksgiving, when retailers urge Americans to shop online). About 100m purchases were logged, accounting for 80% of the packages shipped that day. Couriers(怏递员) were buried in parcels.

So life is good for China's e-tailers, then? Not exactly. The number of digital marketers is increasing and online sales are booming. Consumers are enjoying lower prices, better service and more variety. The problem? The pressure on profits in Chinese e-commerce is worse than in America, reckons Elinor Leung of CLSA, a broker. "Almost no one makes money," she says.

The fiercest battles are being fought between online retailers and their bricks-and-mortar(实体的) rivals.Dangdang, a firm. that resembles Amazon,.and 360buy, another online retailer, have cut prices fiercely. Tencent, a cash-rich online giant known for its instant-messaging software, is splashing out to win market share. 360buy has also just raised $400m from investors to do the same. But it is unclear how much longer such firms can burn through capital.

小题1:What's the best title of this passage?

A.The Ambition of Alibaba

B.Fierce Competition between Retailers

C.A Newly Sprung Festival for the Singles

D.Chinese Booming and Developing E-commerce小题2:According to Zeng Ming, this year Alibaba will        .

A.outweigh Amazon and eBay in worldwide influence

B.rank top among all the Internet firms

C.have more than 159 billion dollars' sale

D.create another sales miracle just like the one on Singles Day小题3:How many packages were shipped on November ll th from Alibaba's online platforms?

A.About 80 million.

B.About 100 million.

C.About 125 million.

D.About 180 million.小题4:What's the author's attitude towards online retailers in China?

A.Optimistic.

B.Concerned.

C.Sympathetic.

D.Indifferent.