Question

A car A moves with the velocity of $20m{s}^{-1}$ and car B with velocity $15m{s}^{-1}$ as shown in the figure. Find the relative Velocity of B w.r.t. A and A w.r.t. B.

Answer 1

Let us assume that the right direction is positive.
We are given: $v_A=20m{s}^{-1}$,$v_B=15m{s}^{-1}$,
Relative velocity of B w.r.t A:$v_{B_{/}A}=v_B-v_A=15-20=-5m{s}^{-1}$ (Negative sign indicates that this relative velocity is in the left direction.)
Relative velocity of A w.r.t B:$v_{A_{/}B}=v_A-v_B=20-15=5m{s}^{-1}$
(Positive sign indicates that this relative velocity is in the right direction.)
Here the separation between A and B is decreasing with time, hence the velocity of approach of A w.r.t B is $5m{s}^{-1}$.