A circle has such a characteristic 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. This will drive more smoothly. This wheel is deformed and runs smoothly.

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!

The round wheel can roll round and round, which is only a superficial phenomenon. 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. The outer circle is called the circumference, and the point to which the compass is attached when drawing a circle 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. 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, it is no longer round, the rim is higher and the rim is lower, that is to say, the distance from the rim to the wheel center is not equal, then this car will go.

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

Then, you must know why you use compasses to draw a circle, because the distance between its feet remains the same after its 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 in the sky and the moon in the half moon are! These objective things make people get the image of a circle, and the concept of a circle gradually emerges. People also began to learn to draw a circle, but it is really not easy to draw a very smooth circle.

From production practice, people know that the distance from all points of a circle to a fixed point (center) is equal, and then they invented compasses to draw a circle.