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在一段东西方向上的平直公路上，有一辆质量m=10T的卡车，正在向东加速行驶，在经过t=50s的时间内，卡车的速度从v1=5m/s均匀增大到v2=15m/s，已知卡车在运动时受到的阻力为车重的k倍，且k=0.05，g=10m/s2．试求：

（1）卡车在这段时间内动能的变化量？

（2）在这段时间里的中间时刻，发动机的瞬时功率？

（3）在这段时间内卡车的牵引力所做的功？

答案

（1）卡车动能的变化量△Ek=

1
2
mV22-
1
2
mV21=106J

（2）汽车的加速度a=

v2-v1
t
=0.2m/s2

F-kmg=ma 解得F=7×103N

中间时刻的速度v=

v1+v2
2
=10m/s

故在这段路程的时间中点发动机的瞬时功率P=Fv=7×104W．

（3）在这段时间内卡车的运动的位移S=

v22-v12
2a
=500m．

所以牵引力所做的功WF=FS=3.5×106J．

答：（1）卡车在这段时间内动能的变化量为106J．

（2）在这段时间里的中间时刻，发动机的瞬时功率为7×104W．

（3）在这段时间内卡车的牵引力所做的功为3.5×106J．

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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.

Which of the following is NOT true about unwanted e-mail

A. It costs money to receive them.
B. It’s free to store them.
C. It takes time to access them.
D. It takes time to throw them away.
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