Three identical balls are thrown from the top of a building, all with the same initial speed. As shown in figure., the first ball is thrown horizontally, second above horizontal level, and third at an angle below the horizontal. Neglecting air resistance, rank the speeds of the balls at the instant each hits the ground.

Explanation

`v_1=v_2=v_3`. The first and third balls speed up after they are thrown, whereas the second ball initially slows down but then speeds up after reaching its peak. The paths of all the three balls are parabolas, and the balls take different time intervals to reach the ground because they have different initial velocities. All three balls, however, have the same speed at the moment they hit the ground because all start with the same kinetic energy and because the ball-earth system undergoes the same change in gravitational potential energy in all three cases.