Why is the horizontal velocity of projectile constant?

Projectile Motion:

1. The projectile motion is the form of motion, experienced by an object when it is projected into the air with some velocity and at some angle.
2. When an object is projected with some velocity (v) and at an angle $\theta$ with the horizontal in the air, there is only one force acting on the object which is the acceleration due to gravity.
3. The horizontal component of the velocity of the object is $\mathrm{vcos}\theta$.

Horizontal velocity of projectile constant:

1. There is only one force acting on the object in the air, that is due to gravity, acting downwards.
2. Since no force is acting in a horizontal direction therefore the magnitude of velocity and angle is remaining constant.

Hence, the horizontal velocity of the projectile is constant.