题干

下列关于运动物体所受合外力做功和动能变化的关系正确的是(   )

A:如果物体所受合外力为零,则合外力对物体做的功一定为零

B:如果合外力对物体所做的功为零,则合外力一定为零

C:物体在合外力作用下做变速运动,动能一定发生变化

D:物体的动能不变,所受合外力一定为零

上一题 下一题 0.0难度 选择题 更新时间:2013-05-21 04:23:55

答案(点此获取答案解析)

A

同类题4

阅读理解

    When I was eleven years old, God gave me a gift of happiness and I had a carefree childhood in France.

    One day, on my way home from school, a truck ran over me and my legs were cut off. As I lay there bleeding, I suddenly realized that I might have to live the rest of my life without legs. How depressed I was at that moment! But then I realized that depression wouldn't get my legs back. So I decided not to waste my life feeling depressed. When my parents arrived at the hospital, they stood there quietly, looked so shocked and sad. I told them, “I've already accepted the fact. Now you also have to get used to this.”

    Ever since then, I often see my friends getting worried about little things: their bus comes late; they get bad grades on a test. But I just enjoy life.

    I am lucky to realize that it is a waste of energy to focus on what you have lost, and that the key to happiness is to take pleasure in what you have. Sounds simple, doesn't it? So why are many people unhappy?

    People often think happiness is based on what they achieve. “My whole life would improve if I had a new car.” But when you get the car and what happens? For a whole week you are walking on air. Then you go right back to being unhappy.

    Happiness is not a happening; it's a state of mind. Even though you have the whole world and you'll still be unsatisfied. And you have relatively(相对地) little but feel full of joy. Happiness comes from mastering the art of enjoying and taking pleasure in what you already have.