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.

Homework should be done as soon as you get home from school. If you have after-school activities, the homework should be done first. At the same time, it is fine to allow（允许）for a snack break and also to allow for stretching(拉伸的)breaks.
Also, you should remember that it is your duty to do your homework. Next time, when you want to depend on(依靠) Mom and Dad for help, please try to find the answer yourself, because you will learn much, much more that way.
When you have a question, especially something about a science or math, don’t always turn to your teacher! It is a good idea to try to find the answer yourself. If this doesn’t work, you can also look at the materials in the back of the book. They may guide you to the pages you should look at to help you understand the problem.
Finally, while doing homework, don’t think about or do other things. Keep working. Setting a time limit(限制) often helps with this. Different ages decide different amounts of time. 10-15 minutes is good for a child that’s aged 6-8. A time of 20-30 minutes is about right for a child aged 9-12. Kids can keep studying for 30-45 minutes from the ages of 13 to 15. This, of course, depends on your abilities to pay attention and stay on tasks.
【小题1】In the first paragraph, the writer mainly tells us that ______.