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

That’s the last one flight of the day and the plane is pretty empty, so you think maybe you’ll just move up a few rows to a nice window seat with a view that doesn’t obstruct the wing.

Not so fast, buster!. The flight attendant says it’s a no go. You have to stay in your assigned seat or you’ll mess up the weight distribution of the plane. Is it going to make any difference? Yeah, you know where it’s going. Answering this question requires some cool physics, so let’s get to it.

Mass center

People often say that the center of mass of an object is where all the gravitational force acts. This is a fair working definition, and you can use it to solve many problems in physics. But in reality, gravitational force is pulling everyone parts of an object, not just a single point.

(Quick side note: We’re actually going to look 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 it torque. Looking back at Newton’s second law, it states that a net force changes the motion of an object (F:_network = mass × acceleration) So if the net force is zero, the motion of the object will not change. If it is moving at a certain speed, it will continue to do so. If it is at rest, it will continue to rest.

Here is a small experiment. Place a pencil on a flat table and then, taking your two index fingers, push it right into the center, right? Since you’re applying equal and opposite forces, what if you? click on it like this.

A pencil pusherGraphics by Rhett Allen