Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won’t fall easily. Then we say that its center of gravity is low.

Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).

Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

1.If the bottom of an object is ______ its top, it will fall over easily.

A. lighter than　　 B. heavier than　　 C. no heavier than　　  D. as heavy as

2.Please point out which box will not fall.

3.The main idea of the passage is _________.

A. why things fall over　　　　         B. a low center of gravity

C. a high center of gravity　　             D. that everything has weight

As we all know, the ocean moves. But waves breaking on the beach make the ocean fun. When you stand on the shore and look at the waves, it seems like the water is moving towards you. But the water is not really moving forward. The moving you see is wave energy. And wave energy comes from the wind.

Let’s think about how this works. As wave energy passes through the water, the energy makes particles （粒子） of water move up and down. Just give you an example of a wave passing under a basketball. We put a basketball on the sea. When the wind blows, a wave comes from behind the basketball. The basketball moves up to the top with the wave. Then the wave continues and leaves the basketball behind. The basketball looks lower, but it doesn’t move forward. That’s because the wave energy just moves the water which is under the basketball.

　  So why can you see a wave knock you down on the shore？When a wave moves toward the shore, the bottom of the wave slows down because it meets the ocean floor. But the top, which is called crest(浪峰), keeps moving. The different action causes the wave to fall over or break onto the shore. If a wave is very large, it carries a lot of energy when it hits the land. As time goes by, waves can break up and carry away the rocks and other things on the shore. Shorelines are changing all the time because of wave action.

1.______ is really moving forward when you look at the wave.

A．Water            B．The basketball      C．Water and wave energy   D．Wave energy

2.Match the sentences with the pictures and the right answer.

(1) The basketball moves up to the top with the wave.

(2) The basketball continues and leaves the basketball behind.

(3) When the wind blows, a wave comes from behind the basketball.

A．a—(2) b—(1) c—(3)                     B．a—(3) b—(1) c—(2)

C．a—(1) b—(3) c—(2)                     D．a—(3) b—(2) c—(1)

3.Which of the following is NOT true according to the passage?

A．Wave energy comes from the water.

B．Large waves carry plenty of energy.

C．Wave action makes shorelines change all the time.

D．The bottom of the wave is easy to slow down.

4.The underlined word “it” ( Paragraph 3) refers to “________”.

A．particles of water   B．the shore          C．the wind          D．the bottom of the wave

5.The main idea of this passage is ______.

A．what makes particles of water move         B．how a basketball moves on the ocean

C．how wave energy causes wave action        D．why a wave can knock you down

In China , more and more middle school students are getting shorter sleeping time than before. Most students sleep less than nine hours every night, because they have much homework to do. Some homework is given by their teachers, and some by their parents. Also, some students don't know how to save time. They are not careful enough while they do their homework, so it takes them a lot of time. Some students spend too much time watching TV or playing computer games. They stay up (熬夜) very late. Some students have to get up early every morning on weekdays to get to school on time by bus or by bike. It can be a long way from home to school.

　　Schools and parents should cut down some of the homework so that our children can enjoy more than nine hours of sleep every night for their health. For children, we should make best use of our time. When we have enough time for sleeping, we will find it much better for both our study and health.

1.What makes most students sleep less than nine hours every night?

　　    A. Watching TV.

B. Playing computer games.

　　    C. Doing their homework.

D. Doing housework.

2.Who else gives children some homework to do except their teachers?

　　   A. Their parents.　　　　　　　　　　　B. Other students.

　　   C. The children themselves.　　　　　　 D. Nobody else.

3.How many hours of sleep is good for students' health?

　　  A. Less than 9 hours.　　B. Over 9 hours.　　

C. Only 9 hours.　    　D.8 hours.

4.What would happen to a student who often goes to bed late and gets up early?

　　   A. He would fall asleep in class.

B. He would do well in study.

　　   C. He would finish his homework fast.

D. He would have good health.

5.Which of the following is NOT true?

　　   A. Some students are so careless that they spend lots of time on their homework.

　　   B. All the students stay up late for their homework.

　　   C. Children should be given less homework.

   D. Schools and parents should take care of children.

Every object has weight. But the center of its weight is not always in the middle of the object. If the top of an object is heavier than its bottom, it will fall over easily. We say that its center of gravity is high. But if the bottom is heavier, it won’t fall easily. Then we say that its center of gravity is low.
Look at the first picture. This empty box must fall over, because its center of gravity (C) is not above the line of its bottom (AB).
Now look at the second picture. There is some heavy iron in the bottom of this box. It will not fall, because its center of gravity is above its bottom.

【小题1】If the bottom of an object is ______ its top, it will fall over easily.