hc verma BAROMETER QUESTION

1) A barometer kept in an elevator reads 76cm when it is at rest.If the elevator goes up with increasing speed,the reading will be,
(a)0 (b)76cm (c)<76cm (d)>76cm

2) A barometer kept in an elevator reads 76cm when it is accelerating upwards.The air pressure in the elevator is
(a)76cm (b)<76cm (c) >76cm (d)0

n a barometer there is a vaccuum above the fluid in the barometer
The atomsphere exerts a pressure at the bottom of the vessel containing the fluid and pushing upto the extent it can
what is this extent
the extent is the point till can counter gravity
when the force exerted by gravity is equal to the force exerted by the fluid
since the area is A is the same we will consider the pressure
P_a = rho*h*g
Above I have explained the principle behind a barometer
so when the liquid is moving upward g_effective =g+a
what does this mean
this means that the liquid is heavier than before
since the atomspheric pressure is not changing there is no extra force which can lift up the liquid upto the same level
the effective pressure of the liquid makes it diffcult for the air to push to the same mark. The air can push only to some mark below
76cm

Now the second case
the g_effective =g-a
this means the fluid is lighter now therefore the air can easily push it over 76cm, because now the
so the reading is greater than 76cm