8 (iii), we have
AC
AB
BC
′
′
=
+
uuuur
uuur
uuuur  = AB
( BC)
+ −
uuur
uuur
a
b
=
−
r
r
The vector AC′
uuuur  is said to represent the difference of 
aand
rb
r Now, consider a boat in a river going from one bank of the river to the other in a
direction perpendicular to the flow of the river Then, it is acted upon by two velocity
vectors–one is the velocity imparted to the boat by its engine and other one is the
velocity of the flow of river water Under the simultaneous influence of these two
velocities, the boat in actual starts travelling with a different velocity