问题 问答题

如图所示,θ=370,sin37°=0.6,cos37°=0.8.箱子重G=200N,箱子与地面的动摩擦因数μ=0.30.要匀速拉动箱子,拉力F为多大?

答案

对木箱进行受力分析如图所示,建立直角坐标系.由平衡条件得:

竖直方向:Fsinθ+N=G

水平方向:Fcosθ-f=0

又  f=μN

联立解得 F=

μG
cosθ+μsinθ

代入解得,F≈61.2N

答:要匀速拉动箱子,拉力F为61.2N.

阅读理解

阅读理解。

     Plants are very important living things. Life could not go on if there were no plants. This is because plants

can make food from air, water and sunlight. Animals and man cannot make food from air, water and sunlight.

Animals get their food by eating plants and other animals. Therefore animals and man need plants in order to

live. This is why we find that there are so many plants around us. If you look carefully at the plants around

you, you will find that there are two kinds of plants: flowering plants and non-flowering plants. Flowering

plants can make seeds. The seeds are protected by the fruits. Some fruits have one seed, some have two,

three or four, and some have many seeds. But a few fruits have no seeds at all. An example of a fruit without

seeds is the banana fruit. Most non-flowering plants do not grow from seeds. They grow from spores (胚芽).

Spores are very small. Some spores are so small and light that they can float in the air. We may say that spores

are quite the same as seeds. When these spores are all on wet and shady places, they usually grow into new

plants.

1. The main idea of the first paragraph is that _____. [ ]

A. plants are important for life

B. plants cannot grow without air

C. there are many plants in the world

D. we can not live without water

2. Plants can make food from _____. [ ]

A. flower, water and air

B. water, sunlight and air

C. air, water and soil

D. air, sun and light

3. What can we infer (推断) from the passage? [ ]

A. Of all living things animals are most important

B. Spores are seeds

C. All fruits of flowering plants have seeds

D. Without plants, man will die out

4. This passage may be taken from _____. [ ]

A. a medicine book

B. a novel

C. a science magazine

D. an experiment report

单项选择题