Frames of reference
A frame of reference is a coordinate system used to measure the physical properties (e.g. position, velocity, momentum) of objects. In physics, the origin of the coordinate system is often the position of an observer.
As there is no known centre of the universe, there is no "correct" frame of reference. The appropriate frame of reference depends on the situation.
A moving object is considered to be stationary in a frame of reference which moves with the same velocity as the object.
A car is moving in the frame of reference of a person standing on the street (i.e. the "perspective" of the person on the street). A person in the car will see the person standing on the street as moving (i.e. the "perspective" of the person in the car).
The magnitude of momentum is strictly relative to the frame of reference:
A car at high speed hitting a stationary wall will be seriously damaged. A car hitting another car going slightly slower in the same direction will be damaged only slightly.
A man walking in the aisle of a plane while flying will have a very small momentum relative to other passengers. However, his momentum is large relative to objects standing on the Earth.