The single insight that unlocks every projectile question: horizontal and vertical motion are completely independent. Horizontally nothing accelerates the object; vertically gravity does. Drag the slider to change the launch angle and watch how each component behaves separately.
When there's no air resistance, the parabolic trajectory is perfectly symmetric. The horizontal velocity component remains constant throughout the entire flight because no horizontal forces act on the projectile. Vertically, gravity decelerates the object on the way up at exactly the same rate it accelerates it on the way down. Time of flight is determined entirely by the vertical motion—solve v = u + at for the upward journey, double it, and you have total flight time. At the peak, vertical velocity reaches zero while horizontal velocity stays unchanged. Range is maximised at 45° because this angle balances time aloft with horizontal speed.
Horizontal and vertical motion are completely independent—they share only time, which lets you solve them as two separate one-dimensional problems.