问题 填空题

如图(1)所示,均匀长方体木块长b=18cm,高h=16cm,宽L=10cm,被两个力传感器支撑起来,两传感器间距为a=10cm且到木块两边的距离相等,传感器能够将支撑点的受力情况通过数据采集器在计算机屏幕上反映出来.现用一弹簧测力计水平拉木块,拉力作用在木块的中点且缓慢均匀增大,木块则始终保持静止状态,计算机屏上出现如图(2)所示的图线.问:

图(2)上的直线A反映的是______传感器上的受力情况(“左边”或“右边”)

弹簧测力计的最大拉力是______N.

答案

以右边传感器为支点,木块保持静止,木块受到拉力F顺时针力矩、左边传感器支持力顺时针力矩,及重力逆时针力矩,根据力矩平衡条件得知,顺时针力矩之和等于逆时针力矩之和,拉力增大,拉力的力矩增大,而重力的力矩不变,则知左边传感器支持力力矩减小,其力臂不变,则左边传感器支持力,根据竖直方向的力平衡得知,右边传感器的支持力

增大,所以直线A反映的是右边传感器上的受力情况.

由图读出,0-5s内,两个传感器读数相等,则知木块的重力G=2×2.8N=5.6N

t=15s,A传感器读数为5.6N.B传感器的读数为0,以右边传感器为支点,根据力矩平衡条件列式

   F

h
2
=G•
a
2

解得,F=3.5N

故答案为:右边,3.5

单项选择题
阅读理解

D  

Every August on the island of Heimaey, young people often take a walk along the street throughout the night. Why their parents allow them to stay out ? The children of Heimaey are going to save young puffins ---- small black-and-white seabirds.

The cliffs(悬崖)above the town are home to a large group of puffins . The birds dig holes all around the cliffs . These holes are their homes. The young stay in the holes for about seven weeks. Then they begin to leave . Some move slowly on their undeveloped wings and feet down to the sea around Heimaey. There they swim, which they can do naturally , and learn to dive for fish . Others, however, may lose their way at night and find themselves in the town instead. On the ground, the young birds are in trouble. Because their wings are fully developed, they cannot take off quickly to sea and safety. The birds then become a good meal for cats and other animals .

For many years, islanders have help the young puffins. At night in late August, children carrying thick paper boxes and lights hurry out to catch puffins . They spend most of the night running after the birds. They put the birds they've caught into boxes and take good care of them .

The next morning the children take the puffins to the sea and set them free. The bird will live at sea until they are at least two years old. Then they will return to the cliffs to build homes of their own.

The children of Heimaey look forward to August !

16.Every August the children of Heimaey are allowed to stay out all night.    

A.to catch puffins and take them home  B.to save puffins from danger

C.to drive puffins away  D.to run after puffins for pleasure

17.The young puffins that lose their way are in the danger of ______.

A.being killed by hunters  B.knowing nothing about the sea

C.being eaten by animals  D.having no fish to eat

18.The children put the seabirds into boxes and ______.     

A.look after them carefully for several days, then set them free

B.set them free when they grow bigger and can take off to sea and safety

C.carry them to the sea right away to set them free

D.they don' t set them free until the next morning

19.The children look forward to August because ______.     

A.they enjoy playing the game of running after seabirds

B.it' s time for them to go out at night to save puffins

C.it' s time for them to catch seabirds

D.they like to stay out at night to catch puffins

20.According to the passage, which of the following is NOT TRUE?   

A.Once a year the children are allowed to stay out at night to have a good time .

B.The earlier home for the young puffins is in the cliffs, not in the sand of shores .

C.Young puffins make their way down to the sea when they are about seven weeks old.

D.The young birds that get to the sea will stay there for at least two years before they return to the cliffs .