The relative velocity (also or ) is the velocity of an object or observer B in the rest frame of another object or observer A. If it is constant,
where is A's velocity in the rest frame of B.
Classical mechanics 
Due to Einstein's Special Relativity (SR), doesn't hold in general. The expression "the velocity of A relative to B" is shorthand for "the velocity of A in the coordinate system where B is always at rest".
Special relativity theory 
- apart from Newtonian limit, velocities are not additive quantities, and
- the difference velocity between A and B is not equal to their relative velocity and particularly has a smaller absolute value. Whereas the maximum difference speed between two objects is 2c, the maximum absolute value of a relative velocity is equal to c.
Joe and Sara are driving in the same direction. Joe’s velocity is 90 km/h and Sara’s 100 km/h. If we take Joe’s velocity as and Sara’s then
This is the velocity observed by Joe. Joe sees Sara moving at 10 km/h.
Joe and Sara are driving in the opposite directions i.e heading towards each other or moving away from each other. Joe’s velocity is 90 km/h and Sara’s 100 km/h. If we take Joe’s velocity as and Sara’s then
This is the velocity observed by Joe. Joe sees Sara moving at 190 km/h.
- Alonso & Finn, Fundamental University Physics ISBN 10:0-201-56518-8
- Greenwood, Donald T, Principles of Dynamics.
- Goodman and Warner, Dynamics.
- Beer and Johnston, Statics and Dynamics.
- McGraw Hill Dictionary of Physics and Mathematics.
- Rindler, W., Essential Relativity.
- KHURMI R.S., Mechanics, Engineering Mechanics, Statics, Dynamics
- Relative Motion at HyperPhysics
- A Java applet illustrating Relative Velocity, by Andrew Duffy
- Relatív mozgás (1)...(3) Relative motion of two train (1)...(3). Videos on the portal FizKapu. (Hungarian)
- Sebességek összegzése Relative tranquility of trout in creek. Video on the portal FizKapu. (Hungarian)