Here's why:
* Gravity: The only force acting on an object in freefall (ignoring air resistance) is gravity. Gravity acts downwards, affecting only the vertical motion.
* Independence of Motion: Horizontal and vertical motion are independent of each other. This means that the object's horizontal speed doesn't influence how fast it falls vertically.
Example:
Imagine you throw a ball horizontally off a cliff. The ball will travel forward (horizontally) at a constant speed, but at the same time, it will accelerate downwards (vertically) due to gravity. The ball will hit the ground at the same time as another ball dropped straight down from the cliff, even though the first ball had horizontal velocity.
Important Note:
This is true in a vacuum. In reality, air resistance can affect both horizontal and vertical motion. The faster an object moves horizontally, the more air resistance it experiences, which can slow it down. This can indirectly affect the vertical motion as well.
