Relative speed (same direction) Two cars travel in the same direction at 9 km/h and 5 km/h. What is the speed of the first car relative to the second?

Introduction / Context:
Relative speed measures how fast one object approaches or recedes from another. For motion along a straight line in the same direction, the relative speed equals the difference of their individual speeds (faster minus slower).

Given Data / Assumptions:

• First car speed = 9 km/h.
• Second car speed = 5 km/h.
• Both move in the same direction on a straight path with constant speeds.

Concept / Approach:
For same-direction motion, relative speed = |v1 − v2|. We take the faster car's speed minus the slower car's speed to obtain the rate at which the gap changes.

Step-by-Step Solution:

Verification / Alternative check:
If you imagine sitting in the slower car, the faster car appears to move past you at 4 km/h. Conversely, from the faster car, the slower one seems to drift backward at 4 km/h.

Why Other Options Are Wrong:

• 2 km/h and 6 km/h correspond to wrong differences.
• 8 km/h is the sum, which applies to opposite directions, not the same direction.

Common Pitfalls:
Using the sum 9 + 5 for same-direction cases; the sum only applies when objects move toward each other.

Final Answer:
4 km/h