The speed of a ball thrown upward and downward will be the same upon striking the ground, neglecting air resistance. Here's why:

* Conservation of Energy: When you throw a ball upward, you give it potential energy (due to its height) and kinetic energy (due to its motion). As the ball rises, potential energy increases, and kinetic energy decreases. At its highest point, kinetic energy is zero, and all energy is potential.

* Gravity's Role: As the ball falls, potential energy converts back into kinetic energy. Gravity acts constantly, accelerating the ball downward. By the time it reaches the same height it was thrown from, it will have the same kinetic energy (and therefore speed) as when it was initially thrown.

Important Note: This assumes no air resistance. In reality, air resistance will slow the ball down on its upward journey and on its downward journey. This means the ball will strike the ground at a slightly lower speed than it was initially thrown.