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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
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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 $$< 76\ cm$$
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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