问题 填空题

如图所示,轻质杠杆OA中点悬挂重为60N的物体,在A端施加一竖直向上的力F,杠杆在水平位置平衡,则力F的大小是     _,保持F的方向不变,将杠杆从A位置匀速提升到B位置的过程中,力F将     _(选填“变大”、“变小”、或“不变”)。

 

答案

30N  不变

题目分析:由题意可知,阻力臂是动力臂的一半,所以由F1 L1=F2 L2可知动力是阻力的一半即为30N。因为动力和阻力的方向不变,所以将杠杆从A位置匀速提升到B位置的过程中,动力臂和阻力臂的比也不变,由F1 L1=F2 L2可知力F将不变。

单项选择题
阅读理解

阅读理解。

     The tea gardens in Sri Lanka are actually large estates (种植园). The best tea usually comes from plants

grown on high grounds, the cool hilltops with rows and rows of tea plants. The plants are about one metre

apart. The plant is often pruned so that it remains only 60 to 90 centimetres high. Pruning is important

because it encourages the growth of tender shoots, or young leaves. It is from these shoots that the best tea

is got.

     Practically all the tea pickers are women. The estates prefer to employ women pickers because they are

more careful. Their thin fingers can easily remove the twin leaves and new shoots from the plant, which are

the parts used for processing (加工) tea. The pickers carry large baskets into which they throw their pickings.

A skilled worker can harvest between 9 to 14 kilogrammes of tea leaves a day. Usually new shoots can be

picked from the plants about every ten or fifteen days.

      Processing tea shoots into the familiar dry tea leaves requires great care and skill. There are various

methods of processing depending on the type of tea required.For black tea,the young green leaves are first

spread out shelves to dry. This process removes much of its water and the leaves become soft. After this, the

leaves are passed through heavy rollers.This operation will press the leaves for juices which give the tea both

its colour and taste.Then the leaves are spread out on floors and left to ferment (发酵) under wet conditions.

Fermentation develops the rich taste of black tea.The fermented leaves are then dried with a hot-air blower

until they become rolled-up black leaves.The final step is to sort and grade them before the black tea is ready

for sale to countries all over the world.

1. In the first paragraph,the word "pruning" means _____.

A. regular cutting of the plants

B. frequent watering

C. regular use of chemicals

D. growing the plants high in the mountain

2. One of the reasons why women are employed to do the picking is that _____.

A. they work harder than men do in the picking

B. they can throw their pickings more easily into the baskets

C. their fingers fit them better for the job.

D. they can more easily find the twin leaves.

3. How many processing steps are mentioned in the last paragraph?

A. Three.

B. Four.

C. Five.

D. Six.

3. What is the writer's purpose in writing the passage?

A. To introduce various methods of tea processing.

B. To persuade readers to buy tea from Sri Lanka.

C. To tell a story that had happened in the tea gardens.

D. To inform readers of tea growing, picking and processing.