This is called fitness iron ball, also called Baoding iron ball, which belongs to one of Baoding's three treasures. General sporting goods or equipment stores sell them. Generally, you can buy it for tens of dollars, and some high-end ones are more expensive.

Fitness ball is one of the traditional folk fitness and health care instruments in China. There are stone, steel, copper, jade and cloisonne fitness balls. Mainly produced in Baoding area, also known as Wang blacksmith iron ball. The reason why fitness ball is beneficial to physical fitness lies in the moderate movement of fingers and palms when playing, which regulates the meridians of the body. This is a handicraft made by using the law of conservation of energy, which is sold in general handicraft shops. The specific name is uncertain because there are too many forms and styles. Newton's pendulum is a desktop demonstration device to demonstrate the law of conservation of momentum. Five spheres with the same mass are fixed by hanging ropes and closely arranged with each other. Newton's pendulum was first proposed by French physicist Eddm Edem Edme Mariotte in 1676. When swinging the rightmost ball and hitting the other four closely arranged balls on the return swing, only the leftmost ball will be ejected.