Topic: Engineering Mechanics
A person walks up a stalled escalator in 90 seconds. When standing on the same escalator, now moving, he is carried up in 60 seconds. how much time would it take him to walk up the moving escalator?
- A. 30 sec.
- B. 36 sec.
- C. 45 sec.
- D. 54 sec.
Answer: B. 36 sec.
Explanation: Let speed of person = u, speed of escalator = v, distance = s. Then $s/u = 90$ and $s/v = 60$. By relative velocity approach, time taken by person to walk up moving escalator = $s/(u+v)$. $\frac{u+v}{s} = \frac{1}{90} + \frac{1}{60} = \frac{5}{180}$. Therefore, $\frac{s}{u+v} = 36$ sec.