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问题 选择题

如图所示，水平板上有质量m=1.0kg的物块，受到随时间t变化的水平拉力F作用，用力传感器测出相应时刻物块所受摩擦力Ff的大小．取重力加速度g=10m/s2．下列判断正确的是（　　）

A．5s内拉力对物块做功为零

B．4s末物块所受合力大小为4.0N

C．物块与木板之间的动摩擦因数为0.4

D．6s～9s内物块的加速度的大小为2.0m/s2

答案

A、在0-4s内，物体所受的摩擦力为静摩擦力，4s末开始运动，则5s内位移不为零，则拉力做功不为零．故A错误．

B、4s末拉力为4N，摩擦力为4N，合力为零．故B错误．

C、根据牛顿第二定律得，6s～9s内物体做匀加速直线运动的加速度a=

F-Ff
m
=
5-3
1
m/s2=2m/s2．f=μmg，解得μ=
f
mg
=
3
10
=0.3．故C错误，D正确．

故选D．

单项选择题

2008年6月5日，A公司向B公司开具一张金额为5万元的支票，B公司将支票背书转让给C公司。6月12日，C公司请求付款银行付款时，银行以A公司账户内只有5000元为由拒绝付款。C公司遂要求B公司付款，B公司于6月15日向C公司付清了全部款项。根据《票据法》的规定，B公司向A公司行使再追索权的期限为( )。

A．2008年6月25日之前
B．2008年8月15日之前
C．2008年9月15日之前
D．2008年12月5日之前
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单项选择题

There is virtually no limit to how one can serve community interests, from spending a few hours a week with some charitable organization to practically full-time work for a social agency. Just as there are opportunities for voluntary service (1) (VSO) for young people before they take up full-time employment, (2) there are opportunities for overseas service for (3) technicians in developing countries. Some people, (4) those who retire early, (5) their technical and business skills in countries (6) there is a special need.

So in considering voluntary or (7) community service, there are more opportunities than there (8) were when one first began work. Most voluntary organizations have only a small full-time

(9) and depend very much on volunteers and part-timers. This means that working relationships are different from those in commercial organizations, and values may be different. (10) some ways they may seem more casual and less efficient, but one should not (11) them by commercial criteria. The people who work with them do so for different reasons and with different (12) , both personal and (13) . One should not join them (14) to arm them with professional experience; they must be joined with commitment to the (15) , not business efficiency. Because salaries are (16) or non-existent many voluntary bodies offer modest expense. But many retired people take part in community service for (17) , simply because they enjoy the work.

Many community activities possible (18) retirement were also possible during one’s working life but they are to be undertaken (19) seriously for that. Retired people who are just looking for something different or unusual to do should not consider (20) community service.

6（）

A.which

B.where

C.as

D.that
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