Alright, so a friend and I have been debating if a change in speed can be negative. I state that speed is final minus initial, which if the inital is low, the the speed will have dropped in magnitude. He states that magnitude is absolute and no negatives are allowed. I was under the belief that you can have a negative scalar, as it conveys an actual drop in the magnitude, whereas a negative vector requires direction. Eg/. A ball is heading towards a wall at 3ms, and rebounds back at 3.5ms, what is it's change in speed?