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Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t 1. On other days, if she remaines stationary on moving esclalator, then the escalator takes her up in time t 2 . The time taken by her to walk up on the moving escalator will be 1) t 1 + t 2 / 2 2) t 1 t 2 /t 2 - t 1 3) t 1 t 2 / t 2 + t 1 4) t 2 - t 1 Now I have solved the question and I am getting answer (2) but answer is (3) as shown is attachment . Dont relative velocities of two body moving in same direction is found out by subtraction.? Please explain.

Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time t1. On other days, if she remaines stationary on moving esclalator, then the escalator takes her up in time t2. The time taken by her to walk up on the moving escalator will be
 
1) t1 + t2 / 2
2) t1t2/t2 - t1
3) t1t2 / t2 + t1
4) t2 - t1
 
Now I have solved the question and I am getting answer (2) but answer is (3) as shown is attachment . Dont relative velocities of two body moving in same direction is found out by subtraction.?
 
Please explain.

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Grade:12th pass

1 Answers

Eshan
askIITians Faculty 2095 Points
4 years ago
Dear student,

\dfrac{d}{t_1}is not the velocity of Preeti(wrt ground), but of Preeti wrt escalator =v_{PE}. This is because it is her velocity when escalator is stationary.

Hence speed of Preeti on moving escalator==v_{PE}+v_E=\dfrac{d}{t_1}+\dfrac{d}{t_2}

Hope it helps. Regards.

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