You are a passenger on a jetliner that is flying at a constant velocity. You get up from your seat and walk toward the front of
the plane. Because of this action, your forward momentum increases. Does the forward momentum of the plane itself decrease, remain the same, or increase?
Assuming the plane and the passenger are in the same system, the total momentum of the system will remain constant. (i.e. net momentum)
This means that to cancel out the effect of the increase in forward momentum of the passenger, the plane will increase its backward momentum (i.e. reduce its forward momentum).
This is an example of Newton's first law of motion:
An object at rest will remain at rest unless an unbalanced force acts upon it. Similarly, a moving object will remain moving in a straight line at constant velocity unless an unbalanced force acts upon it.
Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another.