Question

A mercury barometer reads h₀ as pressure of air in a closed lift at rest. If the lift accelerates up at g/2, what will be :

(a) atmospheric pressure in the lift; and 

(b) the reading of the barometer ?

Answer 1

(a) The air enclosed in the lift will have the same temperature and volume. Its pressure will be the same as P₀ = h₀ ρg. Here ρ is the density of mercury in barometer.

(We shall see in the kinetic theory of gases that pressure is arising due to collision of molecules.)

(b) When the lift accelerates,mercury column also accelerates. Let h be the length of mercury column now, and a (= g/2) be the acceleration.Then, using New ton’s second law of motion, 

Now we compare it with h₀ .

The reading of barometer falls, although pressure in the liftremains the same.