Harry Potter HeWhoMustNotBeNamed engage fight death this time locate opposite end corridor length l two opponent simultaneously charge deadly spell enemy we know impulse Harrys magic spell fly speed p meter second impulse YouKnowWhos magic spell fly speed q meter secondthe impulse move corridor time collision turn round fly cast change original speed then soon impulse get caster wizard reflect send enemy change original speed impulseSince Harry perfectly master basic magic know second collision impulse disappear powerful explosion occur exactly place collision however young wizard is not good math ask calculate distance position place second meeting spell impulse provide opponent change position fight