First off, this isn't homework, I need to re-learn GCSE physics from a textbook and am having trouble with a few aspects of momentum. 1. The problem statement, all variables and given/known data Two people are skating towards each other. The person on the left has a mass of 80kg and is skating at 2 m/s. The person on the right is skating towards the person on the left at 1.5 m/s and has a mass of 60kg At what velocity do they collide and in what direction do they move off in? 2. Relevant equations Momentum = Mass x Velocity 3. The attempt at a solution I multiplied the first skater's mass by their speed and then multiplied the second skater's mass by their speed and added them together to get a combined momentum. Skater One: 80kg x 2m/s = 160 kg m/s. Skater Two: 60kg x 1.5m/s = 90 kg m/s. Total combined momentum of 250 kg m/s The revision guide suggests: 1) Choose which direction is positive? What does this mean? 2) For some reason, they have worked it out as (80kg x 2m/s) + (60kg x -1.5m/s). Why is it minus? I don't understand why one of the speeds is then negative? Thanks in advance.