（2011·福建理综·T29（2））在光滑水平面上，一质量为m，速度大小为
的A球与质量为2m静止的B球碰撞后，A球的速度方向与碰撞前相反。则碰撞后B球的速度大小可能是__________。（填选项前的字母）
A．0.6
B．0.4
C．0.3
D．0.2
选A.
由动量守恒定律得
，规定A球原方向为正方向，由题意可知vA为负值，则
，因此B球的速度可能为0.6v，故选A.
How can we explain the popularity of neckties? For years, fashion historians have predicted that men would 45 wearing ties because they seemed to have little or no function. It may be that men to them because ties As long as world and business. leaders continue to wear ties, the young businessmen will 46 them and ties will remain popular. 47 , neckties do have a different kind of appeal. Neckties cover the buttons of the shirt and emphasize the length of a man’s body. They also 48 color and adornment (装饰)to men’s clothing, which is often plain and serious. Today, there are tens of thousands’ of designs of ties 49 . There are, ties for special 50 he. am for Valentine’s Day, fireworks for the Fourth of July, and Santa Claus for Christmas. There are ties that tell you about a man’s 51 pictures of tennis rackets, motorcycles, airplanes, fishing rods, or musical instruments, to name a few. There are even ties for different occupations such as accountants, firefighter/s, and police officers. And 52 ties are becoming ever more popular. You can get ties covered with colorful frogs and bottles of beer. 53 you enjoy rock music, you can even find a tie in the shape of a guitar! Funny or formal, wide or narrow, it’s 54 that ties are going to be around for a long time.
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