把一圆柱形钢笔套放在嘴边,用力往里吸气,可使笔套挂在嘴边.若已知笔套质量为20克

题型:选择题

问题:

把一圆柱形钢笔套放在嘴边,用力往里吸气,可使笔套挂在嘴边.若已知笔套质量为20克,笔套内横截面积为0.98厘米2,笔套外横截面积为1厘米2,大气压为标准大气压,则笔套内气压至少减少几帕?(  )

A.1.96×103

B.2×103

C.0.96×103

D.9.8×104

考点:压强的大小及其计算
题型:选择题

阅读理解,选择正确答案。

      Mr. and Mrs. Smith come from Sydney. They teach English in a middle school in China. They like their

work. They have a son and a daughter, Jim and Sue. They are all in China now. Mr. Smith can speak Chinese.

He likes swimming and reading. Mrs. Smith likes swimming in the afternoon and cooking. Jim and Sue like

playing chess. They often play games with Chinese boys and girls.

      Jim's uncle, Green, works on a farm near Sydney. He likes swimming, too. He wants to work in China. But

he can't speak Chinese. So he is still there and goes to Chinese classes every week.

1. Where are Jim and Sue from?[ ]

A. America.

B. Canada.

C. Australia.

2. What does Mr Smith like? He likes ___________[ ]

A. cooking.

B. reading.

C. playing games.

3. What does Jim's uncle like? He likes ___________[ ]

A. reading.

B. playing games.

C. swimming.

4. Where does Sue's uncle work?[ ]

A. On a farm.

B. In a school.

C. In a club.

5. Who works in different countries now?[ ]

A. Mr. and Mrs. Smith.

B. Mr. Smith and his uncle.

C. Mr. Smith and his brother.

题型:选择题

依次填入下面一段文字横线处的语句,衔接最恰当的一组是

任何国家在任何时候都不能忽视粮食安全问题。中国多年来,                  

                                      ,              ,                  。

①实现了粮食供应从长期短缺到总量基本平衡、丰年有余的历史性转变

②以占世界7%的耕地养活了占世界22%的人口

③使粮食产量不断攀升

④坚持以自力更生为主的粮食安全战略

⑤推广良种、改善水利条件、精耕细作

⑥在上个世纪末突破5亿吨大关

A.④⑥②⑤③①

B.④⑤③⑥①②

C.⑤①⑥④③②

D.⑤④③⑥②①

题型:选择题

改善民生我们必须要优先发展农业,建设人力资源强国。

题型:选择题

分析下列各种摩擦:(1)走路时,鞋与地面间的摩擦;(2) 骑自行车时,车轮与轴之间的摩擦;(3)汽车行驶时,汽车与空气间的摩擦;(4)皮带传动中,皮带与皮带盘间的摩擦,其中属于有益摩擦的是(  )

A.(1)和(2)

B.(2)和(3)

C.(2)和(4)

D.(1)和(4)

题型:选择题

声振诊断中,振动的基本参数有()

A.时间

B.振幅

C.频率

D.相位

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