Pressure is the amount of force applied to a given area. Pressure is expressed in units such as pounds per square inch in the English system or newtons per square meter in the metric system.
To understand the difference between force and pressure, consider a block of wood one foot on each side and weighing 40 pounds. The force exerted by that block of wood on a table top is equal to its weight: 40 pounds. But the pressure exerted by the wood is the force exerted on each square inch. Since the block of wood rests on an area of 144 square inches (12 inches by 12 inches), the pressure it exerts is 40 pounds ÷ 144 square inches = 0.28 pounds per square inch.
But now imagine that the same block of wood is cut apart and put back together in the shape of a pyramid. And imagine that a way can be found to balance that pyramid of wood on the table top on its point. Then, the pressure exerted by the wood block is quite different. Its weight remains the same, 40 pounds, but all of that weight rests on a single point. Imagine that the area of the point is 0.01 square inch. Then, the pressure exerted by the block is 40 pounds ÷ 0.01 square inch = 4,000 pounds per square inch.
Perhaps you can see why a sharp knife cuts better than a dull one, or why a nail has a sharp point rather than a flat one. The force exerted by the knife or the nail is focused on a small area, creating a large pressure and, therefore, a more effective cutting or driving force.
All forms of matter—solid, liquid, and gas—exert pressure. In the case of solids and liquids, that pressure is caused by the weight of an object and the area on which that weight acts. In the case of a gas, that pressure is caused by the motion of the gas particles. As gas particles travel through space, they collide with walls, table tops, ceilings, floors, and other objects. The collision of the gas particles against these objects causes gas pressure.
The form of pressure best known to most people is probably air pressure. Air exerts pressure, as do all gases, because of the movement of air particles and their collision with other objects. At sea level, this pressure has a value of approximately 760 millimeters of mercury (760 mm Hg). This unit may seem peculiar, but it represents the height of a column of mercury that can be held up by air pressure at sea level.
Atmospheric pressure is also measured in other units such as atmospheres, millibars, inches of mercury, and kilopascals. One atmosphere of pressure is equal to 760 mm Hg; 29.92 inches of mercury; 1013.25 millibars; or 101.3 kilopascals.
Atmospheric pressure depends on a number of factors, including altitude and weather conditions. In general, the higher one goes in the atmosphere, the lower the atmospheric pressure. Also, the greater the humidity (amount of moisture in the air), the lower the atmospheric pressure.