Never, since "vertical" means directly opposing the local gravity field.
Gravity... I think
The vertical velocity is 0. The horizontal velocity is constant during the entire trajectory (and may be zero).
Because gravity is acting on the vertical component, exerting a constant -9.8m/s2 worth of acceleration.
The vertical component of a projectile's velocity is irrelevant. It can be up, down, or zero, makes no difference. As long as projectile motion lasts ... gravity is the only force on the object and you're ignoring air resistance ... its acceleration is constant, and is equal to the acceleration of gravity: 9.8 meters per second2 pointing down.
rainbow trajectory
Gravity... I think
The vertical velocity is 0. The horizontal velocity is constant during the entire trajectory (and may be zero).
Because gravity is acting on the vertical component, exerting a constant -9.8m/s2 worth of acceleration.
The vertical component of a projectile's velocity is irrelevant. It can be up, down, or zero, makes no difference. As long as projectile motion lasts ... gravity is the only force on the object and you're ignoring air resistance ... its acceleration is constant, and is equal to the acceleration of gravity: 9.8 meters per second2 pointing down.
rainbow trajectory
Since the velocity is constant due to the fact that there are no external forces acting in the horizontal direction, if you neglect air resistance, therefore, the horizontal velocity of a projectile is constant.
That combination is called "projectile motion". In the absence of air resistance, its shape is always a parabola.
horizontal
The horizontal component of a projectile's velocity doesn't change, until the projectile hits somethingor falls to the ground.The vertical component of a projectile's velocity becomes [9.8 meters per second downward] greatereach second. At the maximum height of its trajectory, the projectile's velocity is zero. That's the pointwhere the velocity transitions from upward to downward.
A projectile will travel on a straight line unless external forces act upon it. Gravity will pull the projectile downward, i.e. affect its vertical velocity component. This is why the projectile will decelerate upwards, reach a maximum elevation, and accelerate back down to earth. The force vector of air resistance points in the opposite direction of motion, slowing the projectile down. For example, If the projectile is going forward and up, air resistance is pushing it backwards (horizontal component) and down (vertical component). Without air resistance, there is no external force acting upon the horizontal velocity component and the projectiles ground speed will stay constant as it gains altitude and falls back down to earth.
The motion of a projectile is a combination of two motions, a constant speed motion in the horizontal direction, and an accelerated motion in the vertical direction. The velocity component that changes along the path is Vy.
One that goes directly up - the velocity having no horizontal component.