A projectile A is projected from ground. An observer B running on ground with uniform velocity of magnitude 'v' obseryes A to move along a straight line. The time of flight of A as measured by B is T. Then the range R of projectile on ground is

The horizontal and vertical components of initial velocity of projectile are as shown in figure. Since the observer moving with uniform velocity v sees the projectile moving in straight lineHence v = u cosθThe time of flight as measured by observer B T = 2u sin θgHence horizontal range of projectile on groundR = (u cos θ)T = vT