Action and reaction are equal in magnitude and opposite direction by they don't balance each other because they don't occur on the same body. Action is involved on one body and reaction is involved on another body.
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Answer:
The relative velocity of an object A with respect to another object B.
Explanation:
The relative velocity of an object A with respect to another object B is the velocity that object A would appear to have to an observer situated on object B moving along with it.
The instantaneous velocity of the object is its speed and direction at that instant.