Two cars travel in the same direction at 60 km/h and 40 km/h. The velocity of the first car relative to the second is:

Solution

Physics sir · 3 steps

Same direction means you subtract: sit in the second car and watch how fast the first pulls away.

1. Define relative velocity. The velocity of car 1 relative to car 2 is what a person in car 2 sees. Subtract the observer's velocity from the object's velocity.

   v⃗₁₂ = v⃗₁ - v⃗₂

2. Put in numbers. Take the common direction as positive, so v₁ = +60 km/h and v₂ = +40 km/h. Then v₁₂ = 60 - 40 = 20 km/h, along the direction of motion.
3. So. The first car moves ahead of the second at 20 km/h, which is option (b).

The idea to keep. Relative velocity is always v⃗_A - v⃗_B. For motion along one line, speeds subtract when the bodies move the same way and add when they move in opposite directions.

Answer from the NTA final answer key. Solution by PrepWiser.