Two trains approach each other on parallel tracks at 40 km/h and 60 km/h. Their relative speed is:

Solution

Physics sir · 3 steps

Moving towards each other, the speeds add up: 40 plus 60 gives 100 km/h.

1. Pick directions. Take the first train's direction as positive. Then v₁ = +40 km/h, and the second train, coming the other way, has v₂ = -60 km/h.

   v₁₂ = v₁ - v₂ = 40 - (-60) = 100 km/h

2. Check the sense. Sitting in either train, you see the other one rushing at you. In one hour the gap shrinks by 40 km from one side and 60 km from the other, which is 100 km.
3. So. The relative speed is 100 km/h, option (c).

The idea to keep. Relative velocity is v⃗₁ - v⃗₂. Opposite directions make the speeds add; same direction makes them subtract. Fix the signs first, then subtract.

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