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 line

Hence v = u $\mathrm{cos\theta }$

The time of flight as measured by observer B T = $\frac{2\mathrm{u} \sin \mathrm{\theta }}{\mathrm{g}}$

Hence horizontal range of projectile on ground

R = ($\mathrm{u} \cos \mathrm{\theta }$)T = vT