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问题 选择题

如图所示，质量为m的钩码在弹簧秤的作用下竖直向上运动，不计空气阻力．则

A．若钩码做匀速运动，弹簧秤对钩码的拉力和钩码对的弹簧秤拉力一定平衡

B．若钩码做匀加速运动，弹簧秤对钩码的拉力一定等于钩码对的弹簧秤拉力

C．若钩码做匀速运动，钩码的机械能守恒

D．若钩码做匀减速运动，弹簧秤的拉力对钩码做正功

答案

BD

本题考查的是竖直方向弹簧对钩码的作用问题，若钩码做匀速运动，受力平衡，弹簧秤对钩码的拉力等于钩码所受的重力，钩码对弹簧称向下的拉力等于外界对弹簧秤向上的拉力，A错误；若钩码做匀加速运动，弹簧秤对钩码的拉力一定等于钩码对的弹簧秤拉力，B正确；若钩码做匀速运动，钩码的机械能不守恒，除了重力做功还有拉力做功，C错误；若钩码做匀减速运动，弹簧秤的拉力与钩码的位移方向仍相同对钩码做正功，D正确；

单项选择题

习 * * 强调，我国仍处于并将长期处于（），实现中国梦，创造全体人民更加美好的生活，任重而道远，需要我们每一个人继续付出辛勤劳动和艰苦努力。

A、社会主义初级阶段；

B、社会主义；

C、改革开放初级阶段；

D、新民主主义阶段
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填空题



Americans have four primary avenues for making friends: at work, at school, through a hobby or a (1) such as volunteer work and discussion group, or through a family or (2) .
In American culture, a job is more than the work that one performs between the hours of 9 a.m. and (3) . Work becomes a major socializing influence. Many young mothers who (4) their jobs to care for small children remark on the loneliness of their new lifestyles because they have lost their forum for (5) .
A great number of American employers recognize business social (6) and use it to build a family atmosphere on the job so that employees will feel (7) in their work environment. Some employers arrange company get-togethers in their home or at a (8) . American companies have at least one (9) annually, usually at Christmas time. Many corporations have (10) where employees and administrators alike dress (11) to play games such as baseball and volleyball.
Top American administrators often hold social gatherings (12) . Depending upon the size of the employer’s home and the number of (13) . These social gatherings may be picnics, pool parties, or (14) . If the company is large, an employer may hold what Americans call (15) . For an open house, the employer will invite his employees to come to his home (16) between 1 p.m. on a certain day. Those invited usually stay for (17) , chatting with the other guests and their host. People come and go (18) during the designated hours, and the host keeps refreshments, usually (19) or hors d’oeuvres and beverages, available for all who come. For such social gatherings, it is considered (20) the invitation unless you have an excellent reason.
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