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Question

A barometer kept in an elevator reads 76 cm of mercury when the elevator is at rest. If the elevator goes upwards with some acceleration, the reading of the barometer will be

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
When elevator goes upwards with some acceleration, the effective value of acceleration due to gravity increases due to pseudo force acting downwards.
g=g+a ...(i)
When elevator is at rest, the atmospheric pressure measured by the barometer is,
P0=ρgh ...(ii),
where ρ density of mercury used in barometer and h=76 cm

When the barometer is kept in an elevator moving upwards with acceleration a, the atmospheric pressure measured by it will be,
P0=ρgh
h=P0ρg ...(iii)
From Eq.(i) (ii) & (iii),
h=ρghρ(g+a)=ghg+a
Since (g+a)>g, h<h
Reading of barometer will be less than 76 cm

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