Question

Two trains $121m$ and $99m$ in length are running in opposite directions with velocities $40km{h}^{-1}$ and $32km{h}^{-1}$. In what time they will completely cross each other ?

• A. $9s$
• B. $11s$
• C. $13s$
• D. $15s$

Answer 1

Answer: (B)

Here, $v_A=40km{h}^{-1},v_B=-32km{h}^{-1}$
Length of train $A,l_A=121m$
Length of train $B,l_B=99m$
Relative velocity of two trains is given by
$v_{AB}=v_A-v_B=40-(-32)$
$=72km{h}^{-1}$
$=72\times \frac{5}{18}=20m{s}^{-1}$
Total distance to be travelled by each train for completely crossing the other train
$=121+99=220m$
$\therefore$ Time taken by each train to cross the other train
$=\frac{220}{20}=11s$
Hence the two trains will cross each other in $11s$.