Question

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

(a) zero

(b) 76 cm

(c) <76 cm

(d) >76 cm.

Answer 1

Answer ⇒ Option (c).

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 = +ive which is greater than g.

Now, as proved, Pressure is equal, thus,

        hρg = h'ρ(g + a)

    h' = hg/(g + a)

Now, g/(g +a) < 1 (only greater than 1)

thus, h'   < h  < 76 cm.

Hence, Option (c). is correct.