* Ignoring Air Resistance: In a simplified scenario where we ignore air resistance, the only force acting on the balls is gravity. Gravity acts equally on all objects regardless of their mass.

* Conservation of Energy: The initial kinetic energy (energy of motion) of the balls is converted into potential energy (energy of position) as they rise. Since both balls have the same initial speed, they have the same initial kinetic energy.

* Equal Potential Energy: Because both balls have the same initial kinetic energy, they will reach the same height where all that kinetic energy has been converted to potential energy.

Important Note: In reality, air resistance will play a role. The ball with less mass will experience greater air resistance relative to its weight. This will cause it to slow down more quickly and reach a slightly lower height.