Three balls are tossed with same initial speed from a fourth floor dormitory window. ball a is launched 45 degrees above the horizontal, ball b is launched 45 degrees below the horizontal, and ball c is launched horizontally. which ball hits the ground with the greatest speed

All the balls hit the ground with same velocity.  

Explanation:

Let the speed of throwing be u and height of building be h.

Ball A is launched 45 degrees above the horizontal

Initial vertical speed = usin45

Vertical acceleration = -g

Height = -h

Substituting in v² = u² +2as

                       v² = (usin45)² -2 x g x (-h)

                       v² = 0.5u² +2 x g x h

Final vertical speed² = 0.5u² +2 x g x h

Initial horizontal speed = final horizontal speed =  ucos45

Final horizontal speed² = 0.5u²

Magnitude of final velocity

Ball C is launched horizontally

Initial vertical speed = 0

Vertical acceleration = -g

Height = -h

Substituting in v² = 0² +2as

                       v² = 0² -2 x g x (-h)

                       v² = 2 x g x h

Final vertical speed² = 2 x g x h

Initial horizontal speed = final horizontal speed =  u

Final horizontal speed² =u²

Magnitude of final velocity

Ball B is launched 45 degrees below the horizontal

Initial vertical speed = usin45

Vertical acceleration = g

Height = h

Substituting in v² = u² +2as

                       v² = (usin45)² +2 x g x h

                       v² = 0.5u² +2 x g x h

Final vertical speed² = 0.5u² +2 x g x h

Initial horizontal speed = final horizontal speed =  ucos45

Final horizontal speed² = 0.5u²

Magnitude of final velocity

All the magnitudes are same so all the balls hit the ground with same velocity.  

pe = weightxheight = 2000x1000=2,000,000joules