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

  1. t = (u sin α)/f
  2. t = (f cos α)/u
  3. t = (u sin α)
  4. t = (u cos α)/f


After time t,

Velocity = f × t

VBA = (f × t) + (−u) =

VBA2 = (f2 t2 + u2 - 2fut cos α)

For max and min

d/dt(VBA2) = 0

2f2 t - 2fu cos α = 0

t = (u cos α)/f

The correct option is D.