题干

物体沿一直线运动,在t时间内通过的位移为s,它在中间位置
s
2
处的速度为v1,在中间时刻
t
2
时的速度为v2,则v1和v2的关系中正确的是(  )

A:当物体作匀速直线运动时,v1=v2

B:当物体作匀加速直线运动时,v1>v2

C:当物体作匀减速直线运动时,v1>v2

D:当物体作匀减速直线运动时,v1<v2

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答案(点此获取答案解析)

A,B,C

同类题5

阅读理解

    Today we talk about a time when half the world is waking from the dark, cold winter months. Spring! We often describe “spring” as a time of rebirth, renewal and awakening. Many trees are blossoming and early flowers are pushing through the earth. Things are coming to life!

    But the word “spring” is not just a season. It is also a verb that means something going on or coming out quickly. When you put “spring” and “life” together, you will get “spring to life”. This expression means something suddenly becomes very active or perhaps seems more alive! You may “spring to life” after hearing that a distant friend will be visiting you. Or maybe your favorite soccer team finally “sprang to life” in the second half, played well and won the match.

    Now, besides being a season and a verb the noun “spring” refers to a metal coil (线圈) that is wound tightly. When the coil unwinds, it often jumps. So, we often say a person “has a spring in his step” if he is lively and active. He might even appear to jump, or bounce a little when he walks.

    There is another way we use “spring” as a description. In the case of a “spring chicken”, “spring” means young. However, “spring chicken” is also an informal, humorous way to refer to someone who isn't young at all. So, we use this expression in the negative form, as in “no spring chicken”. For example, let's say you know an 85-year-old man who decides to run a marathon, even though he has never exercised before. You could say, “That's amazing! After all, he's no spring chicken.”

    But be careful when using this expression. It could be a little disrespectful. Let's say your boss shows you a picture of his wife, and you say, “Wow, she's no spring chicken.” That response would be disrespectful and a bad career move.