Question

A $100m$ long train crosses a man travelling at $5km/hr$ in opposite direction, in $7.2\text{seconds}$ then the velocity of train is

• A. $40km/hr$
• B. $25km/hr$
• C. $20km/hr$
• D. $45km/hr$

Answer 1

The correct option is D $45km/hr$

Let the velocity of train be $V_T$

Given: velocity of train $V_M=5km/hr=\frac{5\times 5}{18}m/s=\frac{25}{18}m/s$

and time =$7.2s$

Relative velocity= $V_R=V_T+V_M$

$V_R=(V_T+\frac{5}{18})m/s$

Use formula:

$Time=\frac{distance}{speed}$

$7.2=\frac{100}{(V_T+\frac{5}{18})}$

$72V_T+100=1000$

$72V_T=900$

$V_T=\frac{900}{72}$

$V_T=\frac{100}{8}m/s$

$V_T=\frac{100\times 18}{8\times 5}km/hr$

$V_T=45km/hr$

Correct answer is 45km\hr