An up escalator is going at the rate of 10 step/min. How many steps per min should I move down on it as to look like that I am at rest to a person standing on ground.
A
0 steps
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B
20 steps
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C
10 steps
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D
5 steps
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Solution
The correct option is C
10 steps
So in the same time you have to come down by 10 steps so as to remain at the 5th step. So technically after a minute also your displacement is 0. Hence your velocity = 0, w.r.t. a person on ground.
Also, mathematically
= Velocity of lift w.r.t to ground = 10 steps/min
= Vman with respect to ground = 0 as the man's position shouldn't change in time.
Now question asks how much should you move down on a moving lift?
= Velocity of man w.r.t. lift
= vm−vl
= 0 - 10
= - 10 steps/min
This means man should come down 10 steps per minute on the moving lift.