Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity does not change?

Answer

When a body moves in projectile motion it has two components. One of the components is a horizontal component of the velocity and another is a vertical component.

The velocity along the horizontal component does not change because there is no acceleration long horizontal component.  

Whereas velocity along vertical direction keeps on changing because the acceleration due to gravity is acting on the object. At a maximum height of the projectile velocity is equal to zero.

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