Which vector represents the relative motion between your ship and another vessel?

• A. ER vector
• B. EM vector
• C. RM vector
• D. CPA vector

Answer: (C)

Think about how the other ship appears from your own ship’s frame. The vector that describes that motion is the RM vector. It comes from subtracting your velocity from the other vessel’s velocity (v_rel = v_other − v_self), which tells you how fast and in what direction the other ship is moving relative to you. This is the essence of relative motion and is crucial for predicting how the separation between the ships will change over time. The closest-approach concept uses the relative position at the moment of minimum distance, not the ongoing relative motion, so that vector isn’t describing how the other ship moves relative to you. The other labeled vectors don’t represent this relative motion in this context.