问题 计算题

一个做匀加速直线运动的质点,在连续相等的两个时间间隔内,通过的位移分别是24m和64m,每一个时间间隔为4s,求质点的初速度和加速度。

答案

解法一:基本公式法

画出运动过程示意图,如图所示,因题目中只涉及位移与时间,故选择位移公式:

将x1=24 m、x2=64 m、t=4s代入上式解得:a=2.5 m/s2,vA=1 m/s

解法二:用平均速度公式求解

连续的两段时间t内的平均速度分别为:

B点是AC段的中间时刻,则

得vA=1 m/s,vC=21 m/s,

解法三:用△x=aT2求解

由△x=aT2,得

再由,解得vA=1 m/s.

问答题 简答题
单项选择题

Electronic mail has become an extremely important and popular means of communication.

The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects.

The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature.

The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail.

Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or "opt-out" of) receipt of commercial electronic mail from such senders in the future, other senders provide no such "opt-out" mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both.

Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service.

The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well.

It can be inferred from Paragraph 6 that bulk unsolicited commercial e-mail will probably spread ().

A. harmful virus

B. unpleasant news

C. advertisements

D. adult jokes