This is the last time flight during the day and the plane was quite empty. So you're thinking, maybe you'll just move up a few rows where there's a nice window seat with an unobstructed view of the plane's wings.
Not so fast, kid. The flight attendant said that was impossible. You must sit in your assigned seat or you will disturb the weight distribution of the plane. Actually? Does moving a normal sized person make a difference? Well, you know where this is going: Answering this question requires a lot of great physics knowledge. So let's get it started!
Mass Center
People often say that the center of mass of an object is the location where the entire gravitational force acts. That's a reasonable definition and you can use it to solve many physics problems, but it's not really correct. In fact, gravity pulls all parts of an object, not just a point.
(A side note: We'll actually see focusnot 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 need to think torque. Looking back at Newton's second law, it states that a net force changes the motion of an object (Fnetwork = mass × acceleration). Therefore, if the total force is zero, the motion of an object will not change. If it moves at a certain speed, it will continue to move like that. If it is at rest, it will continue to rest.
Here's a little test: Place a pencil on a flat table, then use two index fingers to press from opposite sides, right in the middle. Is it just there? Because you are applying equal and opposite forces, the total force is zero. But what happens if you click on it like this:
Pencil pusherGraphics: Rhett Allain