Question

# A barometer kept in an elevator reads $$76\ cm$$ when it is at rest. If the elevator goes up with some acceleration, the reading will be

A
76 cm
B
>76 cm
C
<76 cm
D
Zero

Solution

## The correct option is C $$< 76\ cm$$Explanation ⇒ Atmospheric pressure will be same in both cases.At Rest $$P = P₀ = hρg$$When it will move upward, then $$P = P₀ = h'ρ(g + a)$$If elevator is moving with increasing speed, which means, that acceleration is positive. Thus, $$g + a = +ve$$ which is greater than $$g$$.Now, as proved, Pressure is equal, thus,       $$hρg = h'ρ(g + a)$$    $$h' = \dfrac{hg}{(g + a)}$$Now, $$\dfrac{g}{(g +a)} < 1$$ (only greater than 1)thus, $$h' < h < 76 cm$$.Hence, Option (c). is correct.Physics

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