Photo: jpmatth (flickr)
Remember those inflatable clowns that stood about three feet high and had big red noses, meant for punching?
Hit the clown as hard as you could, and it would always spring upright again. How did they manage it?
The secret is in its “center of gravity.”
Gravity pulls on you from every point, head to toe, but if you put all those effects together, you can come up with one spot that’s the average. This is convenient, because it turns out any object will behave as if gravity is only pulling on that one spot. That’s your center of gravity.
In people the center of gravity is around your belly button. When you are standing upright, gravity acts as if it were pulling you by a string hanging from your belly straight down between your feet. If you lean to one side, that string hangs to that side as well. When it moves beyond the place where your feet are, you fall over.
The trick with the punch-me clown is keeping its center of gravity low. The clown’s upper body doesn’t have much material. It’s feet, however, have some kind of weight attached, perhaps a dish of sand. That means that no matter how far the clown tips, its center of gravity never moves outside the range of its heavy base. It can’t fall over.
Putting extra weight in the clown’s feet assures that the “pull” will always remain down there. Take away that extra weight and the average will be somewhere else on the body. Punch the clown then and he’ll topple easily.