Final answer:
To double the impact speed of a falling object, you would need to increase the vertical height by fourfold.
Step-by-step explanation:
When neglecting air resistance, if you want to double the impact speed of a falling object, you would have to increase the vertical height by fourfold. This is because the impact speed is directly related to the height from which the object falls. Doubling the impact speed would require quadrupling the height. Neglecting air resistance means that the vertical and horizontal components of motion are independent, so changing the height will not affect the horizontal motion of the object.