1. Compare the speed of falling of an object(比較物體掉落的速度)
First, you need to prepare one paper and a thick book.
-You have two situations.
First, you drop the book and the paper at the same height separately together.
And second, you put the paper above the book and drop them together.
What we do is to observe them how to fall down in two situations.
The right hand side of the picture tells us that if a book and a paper drop together at the same time , the book will drop first, then the paper will drop down slowly. It is due to the air resistance. The paper will be blocked by the air resistance during that situation. And also the book is heavy and the air resistance are not very useful when the book drop down.
But when the paper is put above the book, they will drop together. It is because the force is acting on the same body so that they will drop down together.