题干

下列说法不正确的是(   )

A:物体的动量为零,则物体一定处于平衡状态

B:质量不变的物体,速度不变,则物体的动量一定不变

C:物体的动量不变,则物体的动能一定不变

D:物体的合外力不变,则动量的变化率一定不变

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答案(点此获取答案解析)

A

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阅读理解

    There are more bicycles than residents in the Netherlands,and in cities

like Amsterdam and The Hague up to 70% of all journeys are made by bike.

    The BBC's reporter,Anna Holligan,examines what made everyone get back in the saddle(车座).

    Before World WarⅡ,journeys in the Netherlands were mainly made by bike, but in the 1950s and 1960s,as car ownership increased quickly,this changed.As in many countries in Europe, roads became increasingly crowded.

    The jump in car number caused a huge rise in the number of deaths on the roads.In 1971 more than 3,000 people were killed by motor vehicles,and 450 of them were children.In response a social movement demanding safer cycling conditions for children was formed.Called Stop de Kindermoord,it took its name from the headline of an article written by journalist Vic Langenhoff whose own child had been killed in a road accident.

    The Dutch love of the motor vehicle was also shaken by the Middle East oil crisis of 1973,when oil-producing countries topped export to the US and Western Europe.

    These twin pressures helped to persuade the Dutch government to invest in improving cycling infrastructure(基础设施)and the Dutch urban planners started to change from the road-building policies designed mainly for cars.

    To make cycling safer and more inviting,the Dutch have built a vast network of cycle paths.These are clearly marked,have smooth surfaces,separate signs and lights for those on two wheels,and are wide enough to allow cycling side by side and overtaking.

    Even before they can walk,Dutch children live in a world of cycling.As babies they travel in special seats on bikes.As the children grow up they ride their own bikes.And,as the Dutch are not allowed to drive until 18,cycling offers teenagers an alternative form of freedom.

    The state also plays a part in teaching,with cycling lessons a compulsory(必修的)part in Dutch schools.All schools have places to park bikes and at some schools 90% of pupils cycle to class.