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Question

A barometer kept in an elevator at rest reads 76 cm of Hg. If the elevator starts accelerating downwards, then the atmospheric pressure measured by the barometer (w.r.t column of mercury) in the elevator is:

A
76 cm
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B
<76 cm
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C
>76 cm
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D
zero
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Solution

The correct option is C >76 cm
Let the atmospheric pressure measured by the barometer be P0. The barometer set-up is shown in figure.


In a static fluid, pressure at each point in the fluid at the same level is equal.
i.e Pressure at point A=Pressure at point B
PA=PB
P0=PB ...(i)
Since there is always vaccum,(pressure=0) over the mercury column.
P0=0+ρgeffh=ρgeffh ...(ii)
When barometer is kept in elevator accelerating downwards, geff=ga
P0=ρgeffh
But P0=ρgh
[h=76 cm for barometer reading when lift is at rest]

Equating both,
ρgeffh=ρgh
h=(gga)h
Clearly (gga)>1
h>h
h>76 cm

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