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

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.

Our nearest neighbor is the moon. In the sky,  the moon and the sun seem to be of the same size because the moon is much nearer.
The moon makes the tides—the changes in the level of the sea. The moon and the sun together pull the sea. In some parts of the world,  the difference between * high tide’ (when the sea is very near to the land) and ’ low tide’ (when the sea is far away from the land) is very big. This is very important for ships.
The moon is very different from the Earth. Gravity on the moon is six times weaker than that on the Earth. There isn’t any air. During the day,  it is very,  very hot but at night it is very,  very cold. Nothing can live on the moon.
If the moon goes between the Earth and the sun,  we have an eclipse of the sun(日食) . The moon stops the light from the sun and we have night in the day.
If the Earth goes between the moon and the sun,  we have an eclipse of the moon. The moon goes into the shadow( 影子) of the Earth and it disappears for a few minutes.
【小题1】When the moon goes between the Earth and the sun,  we have night in the day, we call it the eclipse of

Our nearest neighbor is the moon. In the sky,  the moon and the sun seem to be of the same size because the moon is much nearer.

The moon makes the tides—the changes in the level of the sea. The moon and the sun together pull the sea. In some parts of the world,  the difference between * high tide' (when the sea is very near to the land) and ' low tide' (when the sea is far away from the land) is very big. This is very important for ships.

The moon is very different from the Earth. Gravity on the moon is six times weaker than that on the Earth. There isn't any air. During the day,  it is very,  very hot but at night it is very,  very cold. Nothing can live on the moon.

If the moon goes between the Earth and the sun,  we have an eclipse of the sun(日食) . The moon stops the light from the sun and we have night in the day.

If the Earth goes between the moon and the sun,  we have an eclipse of the moon. The moon goes into the shadow( 影子) of the Earth and it disappears for a few minutes.

1. The sun to the Earth is _________times farther than the moon to the Earth.

A. nearly 200         B. less than 300      C. nearly 400         D. more than 400

 2.When the moon goes between the Earth and the sun,  we have night in the day, we call it the eclipse of

A. the moon      B. the sun          C. the stars        D. the Earth

3. Which of the following happens when we have an eclipse of the moon?

A. The Earth goes between the moon and the sun.

B. The moon goes into the shadow of the sun.

C. The moon goes into the shadow of its own.

D. The sun disappears for a few minutes.

4.Which of the following statements are TRUE?

a. The moon and the sun are of the same size.

b. Gravity on the Earth is 6 times stronger than that on the moon.

c. The changes in the level of the sea are called tides.

d. The moon itself pulls the sea and makes the tides.

e. When the sea is near to the land,  a high tide may happen.

f. It is much hotter in the day but much colder at night on the moon.

A. a. b. c. f.        B. b. c. d. e          C. a. d. e. f      D. b. c. e. f