The quantity that has a magnitude of zero when the ball is at the highest point in its trajectory is the vertical velocity.
In fact, the motion of the ball consists of two separate motions:
- the horizontal motion, on the x-axis, which is a uniform motion with constant velocity [tex]v_x=v_0 cos 30^{\circ}[/tex], where [tex]v_0=35 m/s[/tex]
- the vertical motion, on the y-axis, which is a uniformly accelerated motion with constant acceleration [tex]g=9.81 m/s^2[/tex] directed downwards, and with initial velocity [tex]v_y=v_= sin 30^{\circ}[/tex]. Due to the presence of the acceleration g on the vertical direction (pointing in the opposite direction of the initial vertical velocity), the vertical velocity of the ball decreases as it goes higher, up to a point where it becomes zero and it reverses its direction: when the vertical velocity becomes zero, the ball has reached its maximum height.
