A particle projected horizontally with a speed `sqrt2gT` , from a height h above the ground,where T is a constant, moves under gravity.
Draw separate speed-time graphs for the horizontal and vertical components of the velocity of the particle.
The velocity of the projectile has two components. One is horizontal component and the other is vertical component.
When you consider the horizontal component of the velocity has no external forces acting on it. Therefore the horizontal component is constant throughout the projectile.
This is shown in image 1.
When you consider the vertical component of velocity the weight of the particle act on it and it will fall under gravity. So we have a acceleration in the vertical velocity.
This is shown in image 2.