The characteristic of a circle is that the distance from the center of the circle to any point on the circumference is equal, and this equal distance is called radius. In other words: "In the same circle, all radii are equal." Therefore, people make the wheel round so that the axle passes through the center of the circle. When the wheel rolls on the ground, the distance between the axle and the ground is always equal to the radius of the wheel. Only in this way can it run smoothly. If this wheel is deformed, not round, and the distance from the rim to the center of the wheel is not equal, then this kind of car will definitely bump up and down. How can we move forward better?

Of course, the wheel is round because the rolling friction is smaller than the sliding friction.

Why are wheels round? You may say, this problem is not simple, because round wheels roll easily!

It is only a superficial phenomenon that a round wheel can roll round and round. We must grasp the essence of the circle, make a scientific analysis of the circle and find out the fundamental reason why the wheel is round.

What are the important properties of a circle?

Let's look at a circle drawn on the right first. The outer circle is called the circumference, and the point to which the compasses are attached when drawing a circle (now drawn as a black dot for convenience) is called the center of the circle. Let's take a ruler and measure the distance from any point on the circumference to the center of the circle. They are all equal. This equal distance is called radius. This is an important property of a circle.

If the wheel is made into a circle and the axle is placed on the center of the circle, when the wheel rolls on the ground, the distance between the axle and the ground is always equal to the radius of the wheel. Therefore, the car mounted on the axle and the people sitting in the car will be pulled away smoothly by the car. Suppose the wheel of this car is broken, not round, and the rim is higher than the other one, that is to say, the distance from the rim to the center of the wheel is not equal, then this kind of car will definitely knock your head unconscious when walking.

The wheel is round, of course, there are other reasons. For example, it takes a lot of effort to roll things on the ground than to drag them on the ground. This is because the rolling friction resistance is less than the sliding friction resistance.

Then, you must know why you use compasses when drawing a circle. Because the distance between the two feet of the compass is constant after the two feet are opened.

When did people realize the essence of the circle? This was indeed a long time ago. At first, it was nature that inspired people. Look, how round the sun and half moon are in the sky! These objective things give people a round image. The concept of circle gradually came into being. People also began to learn to draw a circle, but it is really not easy to draw a very smooth circle.

From the production practice, people know that the distance from each point on the circumference to a fixed point (center) is equal, so they invented compasses to draw circles.