Answer to Question #259816 in Mechanical Engineering for Jool

Question

A bell jar 250mm in diameter sits on a plate and is evacuated until a vacuum of 700mmHg exists. The local barometer reads 760mmHg. Find the absolute pressure inside the jar, and determine the force required to lift the jar off the

plate. Neglect the weight of the bell jar.

Answer 1

By considering the pressure forces acting on a triangular fluid element of constant depth we can show that the pressure at a point in a fluid in equilibrium (no motion) is the same in all directions; it is a scalar quantity. For gases and liquids in relative motion the pressure may vary with direction at a point; however, this variation is extremely small and can be ignored in most gases and in liquids with low viscosity (e.g., water). We have not assumed in the above discussion that pressure does not vary from point to point, only that at a particular point it does not vary with direction.