In "Princess Pearl", if Sister Rong used such a long needle, wouldn't she be stabbed to death? In addition, a needle with a large stress area cannot be pierced. .

Relationship between skin tension and external force

Of course, the big needle can't get in because of its increased stress surface and small external force. If the external force exceeds the bearing capacity of the bearing surface, it can still enter.

Just like the performance of kung fu, the nail board breaks a big stone. Because of the dense and many nails, the stress surface of the body increases, and the external force laid by the shovel head is released by the stone plate pin on the body, so that the human body will not be injured. This is the relationship between mechanics and external mechanics.