【英语时差8,16】跷跷板-下（在线收听）

There’s a simple way to figure out where the center of mass is. If the big kid is two times as heavy as the little kid, then their mutual center of mass will be two times closer to the big kid than to the little kid. Three times heavier and the center of mass will be three times closer. And so on. So the reason the heavy kid sits on the ground while the light kid is up in the air is that their mutual center of mass is on the heavy kid’s side of the pivot. To make the seesaw work, the heavy kid must push that center of mass forward until it just touches the pivot. Then their average weight pushes right on the middle, letting the ends rock up and down.