Q 11-02-108JEE MainJEE Main 2021 (20 Jul, Shift 2)Easy
A boy reaches the airport and finds that the escalator is not working. He walks up the stationary escalator in time $t_1$. If he remains stationary on a moving escalator then the escalator takes him up in time $t_2$. The time taken by him to walk up on the moving escalator will be:
Answer: (C) $\frac{t_1t_2}{t_2 + t_1}$
Speeds: $\dfrac{L}{t_1}$ (boy) and $\dfrac{L}{t_2}$ (escalator). Together:
$$t = \frac{L}{\frac{L}{t_1} + \frac{L}{t_2}} = \frac{t_1t_2}{t_1 + t_2}$$
Solution by Sreeraj P, M.Sc Physics