Two particles start simultaneously from the same point and move along two straight lines, one with uniform velocity u and the other from rest with uniform acceleration f. Let α be the angle between their directions of motion. The relative velocity of the second particle w.r.t. the first is least after a time

- t = (u sin α)/f
- t = (f cos α)/u
- t = (u sin α)
- t = (u cos α)/f

**Answer**

After time t,

Velocity = f × t

V_{BA} = (f × t) + (−u) =

V_{BA}^{2} = (f^{2} t^{2} + u^{2} - 2fut cos α)

For max and min

d/dt(V_{BA}^{2}) = 0

2f^{2} t - 2fu cos α = 0

t = (u cos α)/f

**The correct option is D.**