Pressure is determined by the flow of mass from a high pressure region to a low pressure region. Pressure measurements are made on the fluid states--liquids and gases. Air exerts a pressure which we are so accustomed to that we ignore it. The pressure of water on a swimmer is more noticable. You may be aware of pressure measurements in relations to the weather or your car or bicycle tires.

What is pressure?

PRESSURE is a force exerted by the substance per unit area on another substance. The pressure of a gas is the force that the gas exerts on the walls of its container. When you blow air into a balloon, the balloon expands because the pressure of air molecules is greater on the inside of the balloon than the outside. Pressure is a property which determines the direction in which mass flows. If the balloon is released, the air moves from a region of high pressure to a region of low pressure.

Atmospheric pressure varies with height just as water pressure varies with depth. As a swimmer dives deeper, the water pressure increases. As a mountain climber ascends to higher altitudes, the atmospheric pressure decreases. His body is compressed by a smaller amount of air above it. The atmospheric pressure at 20,000 feet is only one-half of that at sea level because about half of the entire atmosphere is below this elevation.

Atmospheric pressure at sea level can be expressed in terms of 14.7 pounds per square inch. The pressure in car or bicycle tires is also measured in pounds per square inches. A car should have 26-30 lb/ and bicycle tires 40-60/


The weatherman is likely to give atmospheric pressure or barometric pressure as 30 inches. This type of measurement is made with a Torricelli barometer. It consists of a long tube closed at one end, filled with mercury and inverted in a vessel of mercury as shown in Figure 4.

At sea level, the force of the atmospheric pressure will support a column of mercury 760 mm in height. Actually, the weight of the column of mercury is equal to the force of the atmospheric pressure.

In a similar fashion, atmospheric pressure forces water in a similar column up
to 34 feet high!

Simple Pressure Related Applications:

DRINKING STRAW: A drinking straw is used by creating a suction with your mouth. Actually this causes a decrease in air pressure on the inside of the straw. Since the atmospheric pressure is greater on the outside of the straw, liquid is forced into and up the straw.

SIPHON: With a siphon water can be made to flow "uphill". A siphon can be started by filling the tube with water (perhaps by suction). Once started, atmospheric pressure upon the surface of the upper container forces water up the short tube to replace water flowing out of the long tube.

Boyle's Law:

In 1662 Robert Boyle made the first systematic study of the relationship
between volume and pressure in gases. Boyle's observations can be summed
up in the statement: At constant temperature, the volume of a gas varies
inversely with the pressure exerted on it.


Kinetic Molecular Theory Explanation of Boyle's Law

Observations about pressure may be explained using the following ideas. The rapid motion and collisions of molecules with the walls of the container causes pressure (force on a unit area). Pressure is proportional to the number of molecular collisions and the force of the collisions in a particular area. The more collisions of gas molecules with the walls, the higher the pressure.

Hanging water
Magic Leaky Bottle - bottle with holes
Upside down glass in water
Battle of Two Balloons
Balloon Blown Up inside Bottle
Film Can Space Shuttle
King Kong's Hand