**Motion in the same direction**

When two bodies move in the same direction along a straight line, then the relative velocity of A with respect to B will be equal to the difference between the two velocities.

**Example:** Look at the two figures below. In the first figure two cars are moving in the same direction [Fig (a)] and in the second figure [Fig (b)] they are moving opposite to each other. How will you determine the relative velocity between them? Here, in the case of two parallel linear motion, the following two motions are to be considered for determining relative velocity.

That is, v_{AB} = v_{A} – v_{B}, if v_{A} > v_{B}, then the direction of v_{AB} will be along the direction of v_{A}. So looking from B the body ‘A’ will appear moving in the forward direction with velocity v_{AB}. Again, if it is seen from A the body B will appear to be moving backward with velocity v_{BA} (= – v_{AB}) as the direction of v_{BA} is opposite to v_{AB}.

**Conclusions**

Following conclusions may be drawn from the above example:

(1) If two bodies move in the same direction, then by subtracting their velocities relative velocity can be obtained.

(2) If two bodies move in the opposite direction, then the relative velocity is obtained by addition of the velocities.