How can i show that vi=vf in projectile motion using variables? Could i just say that since its a parabola we know that vx isnt going to change, but that vy is because it is affected by gravity. so when it reaches max. height, and vy=0, you know that half of the flight is done, and that the object will follow the opposite path it took to get up there, to go back downwards resulting in the same velocity since gravity and vx is constant. except i dont know how to show this using variables..