Why can’t they change seats on an empty plane?

is the last flight of the day and the plane is quite empty. So you’re thinking, maybe you’ll just go up a few rows, where there will be a nice window seat with a view that isn’t obstructed by the wing.

Not so fast, friend. The stewardess says that can’t be done. You have to stay in your assigned seat or you’ll mess up the weight distribution of the plane. Actually? Would moving a normal sized human being make any difference? Yes, you know where this is going: answering this question requires a lot of amazing physics. So let’s get to it!

Center of mass

People often say that the center of mass of an object is the place where all the gravitational force acts. That’s a handy definition and you can use it to solve many physics problems, but it’s not actually true. In fact, the gravitational force attracts all parts of an object, not just a point.

(A quick side note: we’re actually going to look at the center of gravitynot the center of mass, but in a constant gravitational field like here on Earth, they are the same).

If you really want to understand the center of mass, you have to think about torque. Going back to Newton’s second law, it says that a net force changes the motion of an object (F_net = mass × acceleration). So if the net force is zero, the motion of an object will not change. If it moves with a certain speed, it will continue to do so. If it is at rest, it will continue to rest.

Here’s a little experiment: place a pencil on a flat table and then, taking your two index fingers, push from opposite sides, right in the center. It just sits there, right? Since you are applying equal and opposite forces, the net force is zero. But what happens if you press it like that?

pencil pusherGraphic: Rhett Allain