The velocity of object 2 relaive to object 1 is given by,
v21 = v2 – v1
The velocity of object 1 relaive to object 2 is given by,
v12 = v1 – v2
The velocity of object 2 relaive to object 1 is given by,
v21 = v2 – v1
The velocity of object 1 relaive to object 2 is given by,
v12 = v1 – v2