A ship currently at position a is sailing on a straight course with velocity u. Simultaneously a submarine at position b manoeuvres to intercept the ship. If the ship maintains its straight course at all times, show that the intercept course is orthogonal to the direction (a - b) and that in order to the ship the submarine must have minimum speed
I'm not too worried about this as I have the answers right in front of me; however the bit I am having trouble with is this part:
I don't understand how and any help would be appreciated.