I'm watching a lecture video describing the following situation: There is a red ball and a green ball on a collision course. The green ball moves with a constant velocity 6 m/s due East, while the red ball moves with a constant velocity 2 m/s due west. They are saying that the velocity of the green ball in the frame of the red ball is 8 m/s. I'm not understanding why it's 8. Obviously they are adding the two velocities together, but why? One would think that since green is headed east and red is headed west, that we should subtract the values. What's the reasoning behind this? Thanks.