Question

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

• A. $76cm$
• B. $>76cm$
• C. $<76cm$
• D. Zero

Answer 1

The correct option is C $<76cm$

Explanation ⇒ Atmospheric pressure will be same in both cases.

At Rest $P=P₀=h\rho g$

When it will move upward, then $P=P₀=h^{\prime }\rho (g+a)$

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,

$h\rho g=h^{\prime }\rho (g+a)$

$h^{\prime }=\frac{hg}{(g+a)}$

Now, $\frac{g}{(g+a)}<1$ (only greater than 1)

thus, $h^{\prime }<h<76cm$.

Hence, Option (c). is correct.