Here's why:

* Forces are vector quantities: This means they have both magnitude (how strong they are) and direction.

* Components: We can break down a force into its components along different axes. For example, a force acting at an angle can be broken into horizontal and vertical components.

* Perpendicularity: Since the force is acting *along* the y-axis, it's completely perpendicular to the x-axis. There's no part of the force that acts in the x-direction.

Visualizing it: Imagine a force arrow pointing straight up (along the y-axis). There's no horizontal component to that arrow.