what is difference between absolute velocity and relative velocity

Absolute velocity of a object is the velocity observed seen as same in every inertial frames of reference.

Relative velocity of an object is the velocity of the object with respect to any other frame of reference.

Suppose a bicyclist move with 6km/hr w.r.t. ground and a train moves 60 km per hour and a pedestrian moves 2 km/hr.

The velocity of the train as seen by the pedestrian will be 60-2 = 58 km/hr while the velocity of the train as seen by the cyclist will be = 60-6 = 54 km/hr.

That is relative velocity of the train w.r.t. the bicycle will be 54km/hr while it is 58 w.r.t. the pedestrian.

But velocity of light or electromagnetic radiation does not depend upon the velocities of other moving bodies. Like the speed of light measured by a man sitting on a space shuttle moving at 30,000 km/hr will be same that of a car moving at 60 km/hr. This is called the absolute velocity.

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