Pressure is defined in as force per unit area. Both parts are equally important in considering the pressure brought to bear an object. A reasonable force delivered to a very small area can produce enormous pressure. Likewise, a small pressure distributed over a wide area can deliver a very large force. Consider a 180 pound man whose shoes are 10 inches long and on average three inches wide. The pressure under his sole if he stands on one foot is 180 pounds divided by 30 square inches which equals six pounds per square inch. If this man tried to walk in the snow field, the pressure six pounds per square inch would easily poke through any crust and our subject would be up to his knees in snow. Wearing snow shoes may well increase the area under his foot from 30 square inches to close to 180 square inches. This reduces the pressure under his fort to one pound per square inch which cannot penetrate the snow quite so deeply. Cross-country skis provide the same outcome for the same reason.

Let us consider what happens when we make the area under a persons foot very small. A woman wearing spike heels (.5 in. x .5 in.)can bring enormous pressures to bear on a would-be assailant's foot by driving her weight down onto his foot. A 120 pound woman can deliver 480 pounds per square inch pressure at the end of one of those heels, more than enough to crush an assailants foot

We also see high-pressure under an ice skater's blade. A properly sharpened blade is given a concave cross-section so that the skate blade will have minimal contact with the ice. The weight of the skater spread over this small area produces a very large pressure on the ice. Water expands when it freezes; keep it from expanding and the water will not freeze. A skater gliding over the ice actually melts the ice, and the ice-momentarily-turned-to-water acts as a lubricant between the steel blade and the solid ice. As the blade moves along, the super cooled water turns to ice once again. this process is called regelation.

Along the same line as the ice skate blade is a demonstration involving a large block of ice, two heavy weights and a length of fine wire that can support the weights without breaking. Attach the weights to ends of the wire and drape this arrangement over the ice. In time the wire will cut its way through the the ice while the block stays in tact because of regelation.

The treads on a tire are present for improved traction but not for the reason you might think. If one were to drag a brick across a horizontal surface, one would encounter the same friction regardless of which side of the brick was touching the surface. That means a tire will have the same amount of friction whether or not it has any tread at all. so, are tire treads a hoax? Not exactly. Treads would be largely unnecessary if we could be guaranteed dry roads all of the time. When a highway is wet, water between tires and roadway has to go somewhere because it is not easily compressable. The space between the treads provides just the answer.

Thtere are many units in use to describe pressure.

1 Pascal (Pa) = 1 N/m^2
1 Lb/in^2 = 1 psi = 6895 Pa
1 mm(Hg) = 133.3 Pa
1 torr = 133.3 Pa
1 bar = 100,000 Pa
1 atm = 101,300 Pa
1 atm = 14.70 lb/in^2