I'm a fresh Calculus $1$ student struggling to make this graph somehow, I've been studying Chapter $1$ from James Stewart's single variable Calculus early transcendentals. Excercise $1.1$ is easy up until this question:
An airplane takes off from an airport and land an hour later at another airport, $400$ miles away. If $t$ represents the time in minutes since the plane has left the terminal building, let $x(t)$ be the horizontal distance traveled and $y(t)$ be the altitude of the plane: Sketch a possible graph of the vertical velocity.
This site draws the graph but I can't understand this, why is this so? Why the graph flips? I'm terrible when physics comes in Math, any formula, I guess $v_0\sin\theta - gt$, would be helpful, in terms of explaining the graph. If this graph is correct, I'm looking for an explanation of the sketch.