Question

Two trains along the same straight rails moving with constant speed 60 km/hr respectively toward each other. If at time $t = 0$, the distance between them is 90 km, the time when they collide is

• A. 1 hr
• B. 2 hr
• C. 3 hr
• D. 4 hr

Answer 1

Answer: (A)

1 hr

Answer 2

The relative velocity $v_{rel.} = 60 - ( - 30) = 90km/hr.$

Distance between the train $s_{rel.} = 90km,$ $\therefore$ Time when they collide $= \frac{s_{rel.}}{v_{rel.}} = \frac{90}{90} = 1hr.$