问题 选择题

在同一平台上的O点抛出的3个物体,做平抛运动的轨迹均在纸面内,如右图所示,则3个物体做平抛运动的初速度VA、VB、VC的关系分别是 (         )

A.VA>VB>VC,tA>tB>tC   

B.VA=VB=VC,tA=tB=tC

C.VA<VB<VC,tA>tB>tC

D.VA<VB<VC,tA<tB<tC

答案

答案:C

考点:

专题:平抛运动专题.

分析:平抛运动在水平方向上做匀速直线运动,在竖直方向上做自由落体运动,落地的时间由高度决定,初速度和时间共同决定水平位移.

解答:解:三个物体落地的高度hA>hB>hc,根据h=gt2,知,tA>tB>tc

xA<xB<xc,根据x=vt知,A的水平位移最短,时间最长,则速度最小;c的水平位移最长,时间最短,则速度最大,所以有vA<vB<vc.故C正确,A、B、D错误.

故选C.

点评:解决本题的关键知道平抛运动的时间由高度决定,初速度和时间共同决定水平位移.

单项选择题
单项选择题

The sudden, dramatic explosion in value of online social media sites like Facebook and Twitter is reminiscent of the rise, about 15 years ago, of the online businesses that created the "dotcom bubble. " The Internet was far less widely used than it is today. Still, visionaries saw the potential for the Internet we have today, so virtual companies sprung up and grew like weeds as investors threw money their way. Some, like Google and Amazon, developed an enduring online presence and lasting financial value. But far too many quickly lost value when it became apparent that their rapid growth wasn’t yielding revenue.

So, how much is Facebook’s network of users really worth The potential is clear—when so many people are gathered in one virtual place, offering so much personal information about themselves, they create an unprecedented platform for targeted advertising. Or they would, if they were on the network to shop. When eBay and Amazon suggest products to their customers, they’re talking to people who’ve already proven that they’re interested in buying similar products. People go on Facebook for a variety of reasons-to catch up with old friends, share pictures, make new acquaintances, and talk, sometimes endlessly, about themselves. Whether they’ll appreciate having their virtual conversations interrupted by advertising, targeted or not, remains unclear.

It’s also unclear whether Facebook will actually be able to share information about its users’ browsing habits with advertisers. Complaints about the ineffectiveness of Facebook’s privacy policies have arisen in multiple countries, part of a larger social concern about how private information gets used on line. In December, the Federal Trade Commission issued a proposed framework that, among other things, would permit Facebook users to block advertisers from accessing information about their online interests. If that framework is implemented and widely used by Facebook subscribers, it could seriously impair the site’s value as a potential platform for targeted marketing.

What is clear is that Goldman Sachs has a significant interest in Facebook’s financial value, at least for the short term. Goldman Sachs’ decision to invest heavily in Facebook has had some interesting impacts. For one thing, the investment has allowed Facebook an opportunity to postpone issuing an IPO. That means that, at least for the moment, Facebook doesn’t yet have to disclose its finances or publicly address investor complaints.

Goldman Sachs’ investment also puts the firm in an ideal position to handle Facebook’s IPO when it eventually is issued, perhaps sometime next year. That, of course, has the potential to generate substantial revenues for Goldman Sachs’ clients. Google’s 2004 IPO raised an initial $1.2 billion for the company. After all the propaganda, Facebook’s IPO can hardly be expected to raise less. However, there remains a significant question as to whether Facebook’s potential for generating income is more virtual than real. If it turns out that Facebook can’t live up to its potential for generating advertising revenue, venture capitalists who invest for the long term may get burned.

One dilemma faced by social media like Facebook is that()

A. its customers often exchange private information on it

B. advertisers often interrupt its customers’ conversations

C. the FTC threatens to limit its service with new regulations

D.the users’ private information is misused by advertisers