近日中国气象局举行新闻发布会称：2013年以来，中国平均雾霾天数为52年来最多，由于雾霾天气能见度低，容易造成汽车追尾事故，假设一条高速公路上，前方车辆因故障突然停车，后车以某一速度匀速行驶，由于雾霾天气能见度低，当后车距离前方故障车115m时，司机才能看见，已知从发现故障车，经操纵刹车，到汽车开始减速所经历的时间（即反应时间）t=0.50s，刹车时汽车受到阻力的大小F为汽车重力的0.45倍，为保证安全，后车匀速行驶时的最高速度不得超过多少？
在反应时间内，汽车做匀速运动，运动的距离s1=vt，
设刹车时汽车的加速度大小为a，汽车的质量为m，有：f=ma，
自刹车到停下，汽车运动的距离s2=
，v2 2a
s=s1+s2，
代入数据联立解得v=30m/s．
答：为保证安全，后车匀速行驶时的最高速度不得超过30m/s．
“The Voice” has become a major player in the TV talent-show stages, as the show has become NBC’s biggest entertainment prime-time program of 2012. In addition to being a hit in the ratings, “The Voice” is also a hit with critics and Emmy voters: The second season of the show , which aired from February to May 2012, has earned the 2012 Critics’ Choice Award for Best Realty-Series Competition and an Emmy nomination (提名) for Outstanding Reality-Competition Program. All the original coaches are back for the third season in a row: Christina Aguilera, Maroon 5 lead singer Adam Levine, Cee Lo Green and Blake Shelton. The show’s other on-air talent returning for Season 3 are host/producer Carson Daly and social media reporter Christina Milian, who has been with the show since Season 2. The grand prize for the winner is also the same: a record deal with Universal Republic Records. The third season of “The Voice” features two big new changes: “steals”, in which the coaches can steal contestants who have been rejected from another coach’s team, and “knockout rounds”. In the “battle rounds,” two contestants face off in a duet (二重唱) of a song chosen by their coach. When a contestant is rejected from a team after a battle round, another coach has the option to “steal” the contestant for his or her team. If more than one coach wants the same contestant, then the contestant gets to choose the coach. In the “knockout rounds,” two contestants face off by each doing a different song of the contestant’s choice, and coaches can no longer “steal” a contestant who is rejected. Season 3 will also have a new group of four advisers, who are each paired with a coach: Green Day lead singer with Aguilera; Mary J. Blige with Levine; Michael Bublé with Shelton; and Matchbox Twenty lead singer Rob Thomas with Green. At an August 2012 press event held at “The Voice” executive producer Mark Burnett’s home in Malibu, California, members of the media gathered for a press conference with Burnett, Aguilera, Green, Shelton, Levine, Daly and Milian to discuss changes in the show and what the future holds for “The Voice.” 小题1:What does the underlined word “aired” in Paragraph 1 mean?
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