Question

Why gravity does not affect the motion of horizontal velocity?

Answer 1

At every point of your motion in a horizontal pathway, the normal reaction from the surface which is always perpendicular to the surface, cancels out the gravity due to Earth, which is always towards the centre of earth and hence your horizontal velocity doesn't get affected.

But in case of motion in an incline the normal reaction from the surface, which is perpendicular to the surface doesn't fully cancel out the effect of gravity, which is always towards the centre of earth and hence you will experience slower velocity in motion along an incline than the motion along a horizontal pathway, even though you started with same velocity in both the cases.