Question

Two trains are moving each other at velocities of 36 km/h and 54 km/h . A passenger in the first train observes that the second train passes his train in a time interval of 6 seconds . what is length of second train ?

Answer 1

Question

Given:

$$\begin{array}{l} v_{1} = 36 \frac{km}{h} = 10 \frac{m}{s} \\ v_{2} = 54 \frac{km}{h} = 15 \frac{m}{s} \\ t = 6 \text{ s} \end{array}$$

Need to find: Length of the second train.

Solution:

The passengers of the first train see that the second train moving with the velocity which is equal to the sum of the first and second trains' velocities: $v = v_{1} + v_{2}$. Then the length of the second train is $L_{2} = v \cdot t = (v_{1} + v_{2}) \cdot t = (10 + 15) \cdot 6 = 150 \, \text{m}$.

Answer: the length of the second train is 150 meters.