【英语时差8,16】跷跷板-上（在线收听）

Remember seesaws? Those things were great. But there was often a problem. Suppose there’s a big kid on one side and a smaller kid on the other side. The seesaw just stays in one position: big kid on the ground, smaller kid up in the air. No fun. After a few different tries, you probably realized that moving the big kid in toward the center made it possible to rock a little. Move the big kid even more and you can achieve balance. Now the seesaw works. What’s going on here? The answer is in the “center of mass.” The center of mass is a point somewhere along the plank that holds the two kids where their average weight falls. If you can get the center of mass to be over the pivot point, the seesaw will be balanced.