Relative Motion on a Straight Line?

📌 CONCEPT: Relative motion is the study of motion of one object with respect to another object when both are moving in a plane.

📐 RULE / FORMULA: The relative speed of two objects moving in the same direction is the difference of their speeds and in the opposite direction is the sum of their speeds.

💡 WORKED EXAMPLE: Consider a man walking inside a train at 5 m/s and the train is moving at 20 m/s. The passerby is moving in the same direction as the train. The relative speed of the man with respect to the passerby is the difference of their speeds, i.e. 20 m/s - 5 m/s = 15 m/s.

⚠️ COMMON MISTAKE: Students often confuse the relative speed when objects are moving in opposite directions. They fail to apply the formula correctly and calculate the sum of speeds instead of the difference.