You are a passenger on a jetliner that is flying at a constant velocity. You get up from you seat and walk toward the front of the plane. As, a result, your forward momentum increases. What (if anything) happens to the forward momentum of the plane?
When you get up from your seat and walk toward the front of the plane, your forward momentum increases. This is because you are adding your own momentum to the general motion of the plane. Momentum can be defined as the product of mass and velocity.
Now, since momentum is a conserved quantity according to Newton's third law of motion, the overall momentum of the system (you and the plane) must also remain constant. Therefore, if your forward momentum is increasing, the forward momentum of the plane must decrease by an equal amount to maintain the overall balance.
This may seem counterintuitive, as it would imply that the plane's speed decreases. However, it is important to understand that momentum is a vector quantity with a direction. When you walk toward the front of the plane, your momentum vector combines with the plane's momentum vector in such a way that the resulting vector is closer to your momentum, causing the plane's momentum to decrease.
So, when you increase your forward momentum by walking toward the front of the plane, the forward momentum of the plane decreases by an equal amount to maintain the overall momentum balance of the system.