问题 选择题

如图甲所示,放在水平地面上的物体,受到方向不变的水平拉力F的作用,F的大小与时间t的关系如图乙所示;物体运动的速度v与时间t的关系如图丙所示.则下列说法中正确的是(  )

A.0~2s内物体不受摩擦力的作用

B.2~4s内物体受到的摩擦力与拉力是一对平衡力

C.2~4s内物体做匀速直线运动

D.4~6s内拉力做功16J

答案

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题目分析:(1)由F的大小与时间t的关系的图象可知,0~2s内物体受到的力,与物体运动的速度v与时间t的关系可知物体的运动情况,物体没有相对运动或相随运动趋势时不受摩擦力;

(2)图象是倾斜线,说明速度在变化,即运动状态在变,物体受到了非平衡力;

(3)根据图象找出4~6s内物体受到的拉力和运动的速度,根据W=Fs=Fvt求出拉力做的功.

A、由乙、丙图象可知,0~2s内物体受到的拉力为0、速度为0,没有相对运动或相随运动趋势,故不受摩擦力的作用,故A正确;

BC、物体做匀加速直线运动,此时拉力不等于摩擦力,不符合二力平衡条件,故BC不正确;

D、在4~6s内拉力为F=2N,速度为v=4m/s,拉力对物体做的功W=Fs=Fvt=2N×4m/s×2s=16J,故D正确.

点评:(1)本题考查学生对图象的认识及力的合成的应用,要求能熟练读图,并能结合运动分析物体的受力情况.

(2)v﹣t图象、F﹣t图象相结合,判断出物体的运动状态,根据平衡状态由物体的平衡条件求出力的大小是本题的关键.

阅读理解

Americans are buying more products and services than ever before through the Internet, and the popularity of online sales is likely to spread to other countries. Online sales now represent as much as one-tenth of all retail(零售) sales in the United States,which has led traditional stores to seek new ways to keep their customers loyal.

Lynne Shaner used the Internet to buy everything she needed for her wedding and holiday gifts for her husband. Other than food, 90 percent of her purchases(购买的东西) were made on her home computer. “I find that, by being able to go online and opt for the things that I need to select and have them delivered to me right at my doorstep, I’m free from all the driving, all the crowds, all the noise, and I usually get a better selection.”

There are a lot of people like her. Experts say American online shopping hit records in both November and December. Fifty-seven percent of Americans have bought something electronically. Store owners worry that this growing amount of online sales will hurt their business. Cornell University marketing professor Ed McLaughlin says, “They should be worried. Anything that can move online will be sold online. And it's just a matter of time. However, traditional stores can keep their customers by selling goods like clothing, which buyers may want to see and try on before purchasing. The stores could also offer things that are difficult to ship. ”

Bill Martin, whose business helps stores learn about their customers, says, “Traditional stores offer a social experience that some people enjoy. There is still a lot of emotion in the buying decision that takes place, and you can't always get that on-line. It's a rather cold process. Traditional stores can provide goods for buyers more quickly than online stores. And some retailers are using websites to persuade people to visit their stores. ”

While online shopping worries some business owners, the only worry for delivery services like FedEx and UPS is keeping up with the number of packages. UPS Manager Dana Kline says her company is very busy at this time of year. UPS is so busy that it has filled 55,000 temporary worker positions during the holiday season.

小题1:According to Lynne Shaner, shopping online ________.

A.helps choose things she needs most

B.will replace traditional shopping soon

C.offer many benefits to customers

D.seldom provides a better selection小题2:What does the underlined phrase “opt for” in the second paragraph mean?

A.choose

B.sell

C.design

D.purchase小题3:Professor Ed McLaughlin thinks that _________.

A.online shopping has little effect on traditional stores

B.it is time for store owners to be anxious about their business

C.traditional stores can do nothing to stop challenges from online shopping

D.traditional stores should sell more things that are difficult to ship小题4:What can we know from the fourth paragraph?

A.Online sales fail to offer customers sense of emotion.

B.Customers enjoy social experience when shopping online.

C.Traditional stores can help customers decide what to buy.

D.Customers can buy goods more quickly after visiting websites.小题5:From the passage we can infer _______.

A.UPS has filled more worker positions than FedEx

B.business owners are concerned about delivery services

C.FedEx is keeping up with UPS on the number of packages

D.delivery services are facing more and more pressure from online sales

单项选择题 A3/A4型题