Pressure
Pressure is defined as the force per unit area, which is calculated by taking the total force and dividing it by the area over which the force acts. Force and pressure are related but different concepts. A very small pressure, if applied to a large area, can produce a large total force.
In English units, pressure is measured in pounds per square inch. Pounds per square foot would also be correct.
In the international system pressure is measured in newtons per square meter, which are defined as pascals abbreviated Pa).( Pressure can also be measured in mm of mercury (Hg). This unit stems from old fashioned barometers that are designed to use atmospheric pressure to lift a column of mercury. The number of mm of mercury is simply the number of mm that the pressure pushes up the mercury column. To convert among these units use 1 mm of mercury = 133 pascals = 0.02 pounds per square inch.
Examples of Pressure
Why does a sharp knife cut more easily than a dull one? A sharp knife blade has a smaller area. Therefore for the same amount of force, the sharp knife blade exerts more pressure and cuts more easily.
A woman wearing spiked high heel shoes can damage a floor more easily than when she is wearing flat shoes. With the spiked heels, the force of her weight is concentrated on a smaller area to exert more pressure.
Blood pressure is the force that the blood exerts on a unit area of our artery walls. If the blood pressure is too low, there is not enough force to push the blood to our brains and we pass out. If this pressure is too high, it can burst a weak blood vessel.
Atmospheric pressure is the force from the weight of the atmosphere above us pushing down on a unit area. In a fluid, which is a liquid or a gas, the pressure increases with the depth of the fluid because there is a greater weight of fluid pushing down on each unit of area. This effect can cause our ears to pop as we go up or down mountains. The atmospheric pressure changes with elevation and we must swallow or yawn to allow
Pascal's principle states that any pressure applied to an enclosed fluid is transmitted to every point in the fluid. This principle is the foundation for all hydraulic devices.
Pressure is a fundamental physical concept with many applications.
Reference: http://physics.suite101.com/article.cfm/understanding_pressure_in_physics
