·A pipe which can be swivelled in a vertical plane is mounted on a cart (see Fig. lE.122). The cart moves uniformly along a horizontal path with speed `v_(1)=2m//s` . At what angle a to the horizon should the pipe be placed so that drops of rain falling plumb with a velocity `v_(2)=6m//s` move parallel to the walls of the pipe without touching them? Consider the velocity of the drope as constant due to the resitance of air.