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.

 

　　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.

 

　　还记得跷跷板吗？它可是伟大的发明，但仍然存在一个问题。

 

　　假设跷跷板的一边坐一个大点的孩子，而另一边坐一个 小点的孩子。跷跷板就只停留在一边：大孩子那边，小孩子则悬在空中。这样就失去了跷跷板的乐趣。几次尝试后，你也许会发现让大孩子往中间坐久可以摇动一点。再让大孩子让中间挪动直至平衡后跷跷板就可以摇动了。

 

　　这到底是怎么回事？答案就是“质心”。质心是跷跷板上支撑两个孩子平均重量的一个点。如果找到了是新的支点，跷跷板就平衡了。

 

　　有一个简单的方法找出质心。如果大孩子的重量是小孩子的2倍，那么质心的位置离大孩子的距离就比离小孩子距离近2倍。重三倍，质心的位置就进三倍，以此类推。

 

　　因此，重的小孩坐在地上而轻的小孩悬在空中的原因是，他们共同的质心在重的孩子这边。要想让跷跷板摇动，重的孩子就得往中间挪直到质心移到中心。然后当他们的平均质量正好在中间，这样两头就可以上下摇动了。