Physics 200 Homework 4, Part Two 1. Stupefy ! At the two ends of a Quidditch field are goal posts. Harry Potter and Ginny, under a powerful spell, are flying towards each other low to the ground. They are on a fatal collision course, with Harry moving at 0.6c and Ginny at 0.8c. They pass the goal posts at the same time (in the ground's frame of reference). Standing between them, right in the middle of the field, Hermione anticipates the collision and casts Stupefy at both of them, at the instant in which Harry and Ginny pass over the goal posts. Spells travel at the speed of light. Does there exists an observer in whose reference frame Harry and Ginny are hit by the Stupefy spell at the same time? if so, what is this observer's speed and direction relative to the ground (ie, is the observer moving towards Ginny or towards Harry)