问题 问答题

在一次实践活动中,某同学分别用以轻质杠杆(O为支点,OA=3OB)、一只滑轮和一些细绳构成的简单机械来提升重90N的物体,如图甲、乙所示.

(1)在不计机械重、绳重和部件间摩擦时,F1=______N,F2=______N.

(2)使用图乙组合机械的好处有______和______.

(3)在进行图甲的实验中,若测出F1为50N,则它的机械效率η=______.

请你提出一条增大机械效率的办法:______.

答案

(1)甲图拉力是重力的一半,F1=

G
2
=
1
2
×90N=45N,

乙图中F1?OB=F2?OA,

所以F2=

F1?OB 
OA
=
1
3
×45N=15N.

(2)很明显45N>15N,所以使用乙图组合机械的好处是可以更加省力;

因拉力的方向向下,使用方便了.

(3)因为甲图是动滑轮,则n=2,所以,s=nh,

则η=

W有用
W 
=
Gh
Fs
=
Gh
Fnh
=
G
Fn
=
90N
2×50N
=90%;

对于甲图动滑轮,增大机械效率的办法是增加提升的物体的重力,或减少动滑轮的重力.

故答案为:(1)45,15;(2)省力;使用方便;(3)90%;增加提升的物体的重力.

选择题
写作题

阅读下面的短文,  然后按照要求写一篇150词左右的英语短文。

     News report: Four young female college students died on Friday after falling or jumping from a

blazing room on the sixth floor of Shanghai Business School. Witness said one woman who jumped

appeared to be aiming for bushes on the ground, but missed and landed on concrete. Witnesses said

the other three were desperately trying to climb to another room amid roaring flames when they fell to

the concrete paving.

     The fire broke out in Room 602 of the sevenfloor dormitory at the school's downtown campus on

Zhongshan Road West, just after 6 am. The fire was caused by an electrical device used to heat water.

This had ignited flammable objects, police said late Friday afternoon. After receiving reports of the fire

at 6: 12 am, police, firefighters and ambulances were quickly on the scene. The fire was extinguished

by 6: 30 am, according to the school.

     There is only one public toilet on each floor to get water and fire extinguishers in the dormitory were

outdated, students said.

[写作内容]

1. 以约30个词概括这篇报道的主要内容;

2. 以约120个词针对此篇报道发表你对校园安全问题的看法,   至少包括以下内容要点:

(1)作为学生,  我们该如何在危急的状况下保护好自己;

(2)作为校方应该采取哪些措施避免此类悲剧的发生。

[写作要求]

1. 作文中可使用实例来支持你的观点,  也可以参照阅读材料的内容,  但不得直接引用原文中的句子;

2. 作文中不能出现真实姓名和学校名称。

[评分标准]

概括准确,  语言规范,  内容合适,  篇章连贯。

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