A $120 ext{ m}$ long train is moving in a direction with speed $20 ext{ m/s}$. A train B moving with $30 \

Question

A $120 	ext{ m}$ long train is moving in a direction with speed $20 	ext{ m/s}$. A train B moving with $30 	ext{ m/s}$ in the opposite direction and $130 	ext{ m}$ long crosses the first train in a time:

Accepted Answer

1.  **Concept - Relative Velocity:** When two objects move in opposite directions, their relative velocity is the sum of their individual speeds. 
    *   $v_{relative} = v_A + v_B = 20 	ext{ m/s} + 30 	ext{ m/s} = 50 	ext{ m/s}$ .
2.  **Total Distance:** For the two trains to cross each other completely, the total distance to be covered is the sum of their lengths.
    *   $d = L_A + L_B = 120 	ext{ m} + 130 	ext{ m} = 250 	ext{ m}$.
3.  **Calculate Time:** The time taken to cross is the total distance divided by the relative speed.
    *   $t = \frac{d}{v_{relative}} = \frac{250}{50} = 5 	ext{ s}$.

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