题干

一辆东风牌汽车发动机功率为160kW,运送5t的货物,以4m/s的速度沿30°角的斜坡匀速到达坡顶,已知斜坡长60m(g=10N/kg),则下列说法错误的是(   )

A:汽车从斜坡底端到达坡顶的时间是15s

B:汽车上坡时的牵引力是4×104N

C:汽车上坡时受到的摩擦力为1.5×104N

D:汽车运送货物的机械效率是50%

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C,D

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    When we think about happiness, we usually think of something surprising and unexpected, a top great delight.

    For a child, happiness has a magic quality. I remember playing police and robbers in the woods, getting a speaking part in the school play. Of course, kids also experience lows, but their delight at tops of pleasure is easily seen, such as winning a race or getting a new bike.

    For teenagers, or people under 20, the idea of happiness changes. Suddenly it's conditional on such things as excitement, love, and popularity. I can still feel the pain of not being invited to a party that almost everyone else was going to. I also recall the great happiness of being invited at another event to dance with a very handsome young man.

    In adulthood the things that bring great joy — birth, love, marriage — also bring responsibility and the risk of loss. Love may not last; loved ones die. For adults, happiness is complex.

    My dictionary explains “happy” as “lucky” or “ fortunate”, but I think a better explanation of happiness is “ the ability to enjoy something”. The more we can enjoy what we have, the happier we are. It's easy for us not to notice the pleasure we get from loving and being loved, the company of friends, the freedom to love where we please, and even good health. Nowadays, with so many choices and much pleasure, we have turned happiness into one more thing we have. We think we own the right to have it, which makes us extremely unhappy. So we try hard to get it and consider it to be the same as wealth and success, without noticing that the people who have those things aren't necessarily happier.

    While happiness may be more complex for us, the answer is the same as ever. Happiness isn't about what happens to us. It's the ability to find a positive for every negative, and view a difficulty as a challenge. Don't be sad for what we don't have, but enjoy what we do possess.