In a recent video, Celtics player Kyrie Irving stands on a basketball court, with his legs straight and feet planted together, and leeeeeeans forward. He looks like he's about to topple. But he doesn't. He calmly returns to a normal upright standing position, shakes out his arms and stretches his neck, and then leeeeeeans a ridiculous amount to the side. Again he holds the position in what looks like a brazen defiance of the laws of physics. So what's going on?

#### Twitter content

To understand how to think about center of mass, we have to start with a few basics.

In physics courses, we often treat objects as "point masses." A point mass has no dimensions. You can describe the location and orientation of a point mass with just three variables—its position in the x, y, and z directions. That's it. This point mass approximation is very nice. It allows us to make a complicated problem just a little bit easier (and more manageable).

If you toss a tennis ball across the room, you can approximate this as a point mass. It doesn't matter if the ball is rotating or not (at least for most cases). There is only one force acting on the ball (the gravitational force) and it doesn't really matter WHERE this force acts on the ball. Anyway, it's just a ball—it's almost a point mass anyway.

Now consider something else. Suppose I place a pencil on a table (you can do this yourself). If I push the pencil near the eraser (or tip), the pencil will rotate. If I push the pencil at the middle or the other end, something different will happen. If you don't have a pencil to try this yourself, this is what it would look like.