Fig.1 describes the method of getting relative velocity.
Let B moves with velocity vB and A moves with velocity vA as shown in fig.1 .
Let θ be the angle between the velocity directions of A and B.
Relative velocity of B with respect to A is obtained from the diagonal of parallelogram
formed by vectors -vA and vB as shown in figure 1.
Angle Φ made by relative velocity vector with -vA is given by, .................................(1)
In the given problem we need to get relative velocity of wind with respect to launcher because the flag in the
launcher will point in the direction of relative velocity of wind.
Fig.2 shows the vector diagram , showing the velocity V of launcher , velocity u of wind and
relative velocity of wind (u-V) with respect to launch.
Angle between velocity vector V of launcher and velocity u of wind is (90-α).
Angle between relative velocity vector (u-V) and -V is (180-β).
Hence as per eqn.(1), we write,