Pressure

"We haven't got the money, so we've got to think"
Ernest Rutherford

Pressure is the "Normal force per unit area", where the normal force is the force applied at right angles to a surface.

If a force, F, acts on a surface of area A, at an angle theta with the vertical the pressure, P, exerted on the surface is given by,

Units

SI - 1 Pascal (Pa) = 1 Newton/m²
British - 1 lb/ft²
Other - 1 Psi = 1 lb/in²

⁵

millibar

^-3

Gauge pressure is the excess pressure above atmospheric pressure.

P_absolute = P_gauge + P_atmospheric

For an uncompressable fluid (rho constant), pressure increases linearly with depth. At depth 'h' in a fluid of density (), pressure due to fluid is given by

This cannot be applied to the atmosphere since the density of the atmosphere is not constant.

Unless prevented to do so fluids will flow from high to low pressure regions.

"A strange thing happened during a performance of Elgar's Sea Pictures at a concert hall in Bermuda tonight. The man playing the triangle disappeared."

Ronnie Barker

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu