It is important to note that the phrase perfectly horizontal velocity of 5.0 m/s implies that the full velocity (5.0 m/s) is directed along the x-axis, with zero y-component for the initial velocity. This phrasing is extremely common in physics. You will also encounter the perpendicular case of a "perfectly vertical velocity". It is also worth remarking that although 5.0 m/s is the velocity at the instant of release (clearly the ball's initial velocity for the freefall trajectory of interest) we have written vx = 5.0 m/s rather than vx,i = 5.0 m/s. This is not a typo, because the x direction is subject to the 1-D Motion with Constant Velocity model (recall ax = 0). Because the x velocity is constant, it does not require labels for initial or final states. |