• A moving object can only have one velocity at any moment when observed by single observer.

• A second observer, moving with respect to the first observer will see the same object moving with a different velocity at that same moment.

• If the second observer is moving at a constant speed relative to the first ovserver, then the velocity at which the object is moving in both frames are related by simple vector addition.

• Relative velocity vector addition involves adding two vectors that are not in the same frames of reference that produces a third vector in one of the frames. Most the other equations such as those for projectile motion only involve one frame.
  
  
• The "super observer frame of reference" chosen for the above equations is one where observer A is at rest.
  
  
• Relative motions problems are most often asociated with an object that is moving in a medium B such as air or water in which the medium itself is also moving. The velocity of the object relative to the medium v_O/B is same as the object's velocity in still air or water. It is also the velocity of the object measured by an observer B that is moving along with the medium. The velocity v_B/A is the velocity of the object observed from the rest frame A.
  
  
• The velocity of observer A relative to observer B is the negative of the velocity of observer B relative to Observer A,

An frame of reference moving at a constant speed is called an inertial frame.

Relative Motion of a River Boat
The heading and speed of a river boat (relative to the river flow) can be changed. This is a vector addition problem of the velocity heading of the boat and the velocity of the river. The resulting vector is the actual motion of the boat relative to the stationary bank.