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问题 选择题

如图所示，滑轮组拉物体A在水平桌面上做匀速直线运动。如果拉力F=3N，物体A的速度v=0.2m/s，忽略滑轮重、绳重和轴摩擦。下列说法正确的是：

A．物体A受到合力不为零；

B．物体A受到的拉力为9N；

C．竖直墙受到的拉力为9N；

D．拉力F每秒钟做功为1.8w

答案

答案：B

题目分析：如图所示，滑轮组拉物体A在水平桌面上做匀速直线运动。如果拉力F=3N，那么物体受到的摩擦力为9N。因为物体A在水平桌面上做匀速直线运动，所以物体A受到合力为零；此时它受到的摩擦力和拉力是平衡力，物体A受到的拉力为9N；物体A的速度v=0.2m/s，拉力运动的速度是0.6m/s，拉力F的功率是P=FV=3N*0.6m/s=1.8W；拉力F每秒钟做功为W=Pt=1.8W*60s=108J。

A．物体A受到合力不为零； 不合题意；

B．物体A受到的拉力为9N；符合题意；

C．竖直墙受到的拉力为9N； 不合题意；

D．拉力F每秒钟做功为1.8w 不合题意；

故选B。

点评：在此题中注意承担物体摩擦力绳子的段数是3段，计算拉力的功率时要用拉力的大小和拉力运动的速度计算。

选择题

某行业去年的劳动生产率是每小时生产1件商品，其价值用货币表示为150元。假定甲生产者今年的劳动生产率是每小时生产4件商品，而该行业的劳动生产率提高200％。在其他条件不变的情况下，甲生产者今年1小时内创造的价值总量用货币表示为[ ]

A．200元

B．100元

C．400元

D．250元

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单项选择题

D


An Englishman named Phil Shaw invented extreme ironing in 1997 because he was bored doing ironing at home.
Extreme ironing is a special kind of outdoor sport. It combines (使结合)danger and excitement with practical ironing.
Who does extreme ironing People all over the world who like danger do it. There are about 1000 people around the world who do extreme ironing. There are extreme ironing clubs and groups in Britain, Germany, South Africa, New Zealand and the United States. You don’t have to have any special qualifications(资格证明)to do it. In fact, anyone can do it. However, it is better to start ironing in your back yard(院子) before you start to iron on top of a mountain.
There are, of course, some rules in extreme ironing. The ironing must be outdoors. You must take an iron, an ironing board, and something that you actually iron to get the wrinkles out. Outdoors includes faraway and unusual places, such as on top of a mountain, while skiing, snowboarding, skydiving, or under water. There are no limits. The more unusual the better. However, you should plan carefully because weather conditions can suddenly change on mountains, for example. You should not do extreme ironing on your own.
What kind of things should you have Your must have a real iron. Travel irons are acceptable. You must not use plastic or toy irons. The clothes you iron must he at least the size of a hand towel. It shouldn’t be just a handkerchief. The board doesn’t have to be a full-size board, but that is preferred. It must be one meter long and thirty centimeters wide and have legs. You must also have proof (证据) of ironing that is either a photo or a video, and it should show a recognizable landmark. Be aware (意识) that people have already ironed near the top of Everest, the highest mountain in the world, and in the Blue Hole in Egypt, one of the most dangerous places to dive. However, there are still many records to break. Iron on!

Which is the best advice for beginners to do extreme ironing ( )

A．Doing extreme ironing alone.

B．Beginning ironing in their back yards.

C．Starting ironing on top of a mountain.

D．Joining extreme ironing clubs or groups first.
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