Assuming that the density of air is a constant 1.3 kg/m3 and that the air pressure is 30 inches (76 cm) of Hg, what is the pressure at the top of a 500 meter high skyscraper?

Question

anonymous · Answer

It is atmospheric pressure minus rho g h,
rho = air density
g = 9.8N/kg
h = height in meters

shamim · Answer

@kacy83

shamim · Answer

do u need any more explanation @kacy83

mrnood · Answer

What a TERRIBLE question.

Why is any serious physics course working in inches Hg?

I appreciate it is a historic measure - but it is NOT a measure of pressure - it is an indicator (it doesn't have the correct dimensions for Pressure)

The question should at least state (although it is assumed) that the first measure is at ground level.

After you have worked out the reduction in air pressure, in N/m^2 you will need to convert this to equivalent inHg to give the answer in their units.

I think you need density of Hg to do this.

mrnood · Answer

measuring with a mercury barometer is essentially a comparator of pressures.

The pressure due to the height of the mercury column balances the pressure of the air.

Pressure of a column (assuming incompressible - as stated in question) = rho g h

So the change in pressure of the air is rho(air) g 500
And change in pressure of mercury column = rho(Hg) g (h2-h1)

you are asked to find (h2-h1) (i.e. the change in mercury column)

so you need rho (air ) (given) , AND rho (Hg)  look it up.