The amount of force that presses on a certain area is known as pressure. The pressure on the surface will increase if you make the force on an area bigger. Making the area smaller and keeping the force the same also increase the pressure.

Question: You can push a drawing pin into a piece of wood- but you cannot push your finger into the wood even if you exert a larger force. Why? What is the difference between a sharp knife and a blunt knife?

Answer: The difference in each case is a difference of area- the point of the drawing pin and the edge of the sharp knife have a small area. A force acting over a small area gives a large pressure. Pressure is force per unit area, or pressure= force (in Newton)/ area (in square meters). Its unit is newtons per square meter/m2). The unit is also called the Pascal (Pa), named after Blaise Pascal, who investigated air pressure.

Pressure = distribution of force over an area!

P = F/A units: N/m2

Units Of Pressure

• Pounds per square inch (PSI)

• Newton force per square meter / Pascal (Pa); where kPa is commonly used

• Kilogram per square centimeter (kg/cm2)

• Inch of water (inH2O)

• Millimeter of water (mmH2O)

• Inch of mercury (inHg)

• Millimeter of mercury / torr (mmHg)

• Atmosphere (atm)

• Bar

Differential Pressure
Differential pressure is the amount of pressure drop across a measuring device. This is measured at specific tapping point locations. Flow measurement based on differential pressure readings is one of the most commonly used for the flow of gas and liquids. An orifice plate is an example of a differential flow element. The value of differential pressure represents a small fraction of the actual flowing or static pressure. The differential pressure may be 10kPa but the static pressure may be as high as 800kPa.

Dynamic Pressure
In a pitot tube or annubar, the total pressure and static pressure are measured The dynamic pressure is calculated by subtracting the static pressure from the total pressure. The dynamic pressure is related to the velocity of the fluid. At zero velocity the static pressure acting on the tip of the pitot tube and static pressure holes is equal. No difference in pressure is observed. As the flow velocity increases, the pressure detected at the tip of the tube increases in proportion to the square of the velocity. This difference in pressure is due to dynamic pressure.

Static Pressure
Static pressure is the actual pressure exerted by a fluid or gas. Static pressure is constant. I.e. remains the same whether the gas is moving or not.

Total Pressure
Total pressure is the sum of the static pressure and the dynamic pressure.

Pressure Loss
Pressure loss is the differential pressure in a flowing fluid stream between the inlet and outlet of:-
A flow meter, A flow straightener, Control valve, Length of pipe, etc.

Definitions of pressure on the Web:

• the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit); "the compressed gas exerts an increased pressure"

• a force that compels; "the public brought pressure to bear on the government"

• press: the act of pressing; the exertion of pressure; "he gave the button a press"; "he used pressure to stop the bleeding"; "at the pressing of a button"

• imperativeness: the state of demanding notice or attention; "the insistence of their hunger"; "the press of business matters"

• the somatic sensation that results from applying force to an area of skin; "the sensitivity of his skin to pressure and temperature was normal"

• coerce: to cause to do through pressure or necessity, by physical, moral or intellectual means :"She forced him to take a job in the city"; "He squeezed her for information"

• an oppressive condition of physical or mental or social or economic distress

• blackmail: exert pressure on someone through threats

• Pressure is the application of force to a surface, and the concentration of that force in a given area. A finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall, even though the force applied is the same, because the point concentrates that force into a smaller area.

• The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

• the exertion of force upon a surface by a fluid (eg, the atmosphere) in contact with it.

• The force exerted on a surface divided by the area of the surface.

• Force measured per unit area. Absolute pressure is measured with respect to zero pressure. Gauge pressure is measured with respect to atmospheric pressure.

• Pressure is a similar idea to stress, the force intensity at a point, except that pressure means something acting on the surface of an object rather than within the material of the object. When discussing the pressure within a fluid, the meaning is equivalent to stress.

• force per unit area.

• The force per unit area in a gas or liquid, typically expressed as pounds per square inch in the US. The scientific unit is newtons per m 2 (also known as the pascal). For instance, atmospheric pressure at sea level on Earth equals 14.7 lbs./in 2 = 1.01 x 10 5 newtons per m 2 . The pressure at the center of the Sun is approximately 300 billion Earth atmospheres, or 3 x 10 16 newtons per m 2 .

• The force exerted against an opposing body or the thrust distributed over a surface, expressed in weight per unit of area. Absolute - The pressure above zero pressure, the sum of the atmospheric and gauge pressures. Atmospheric (Standard) - The pressure of the weight of air and water vapor on the surface of the earth at sea level, namely 29.92 inches (760 mm) mercury column or 14.69 pounds per square inch (101.3 kPa).

• action of a defender to restrict the time and space available to an offensive player.

• A force that is applied uniformly to a surface, measured as force per unit of area.

• Force per unit area, measured in interior ballistics terms of pounds per square inch.

• Very low tyre pressure (about 60% of normal road pressures), always associated with a low maximum permitted speed (20 kph or 12 mph) used for traversing or recovery from very soft ground. Such low pressures cause extreme tyre sidewall flexing - hence the speed limitation.

• The force exerted by the weight of the atmosphere, also known as atmospheric pressure. When measured on a barometer, it is referred to as barometric pressure and it is expressed in inches of mercury, millibars, or kiloPascals.

• Atmospheric pressure at the Earth's material surface, measured in millibar (mbar), or hecto Pascal (hPa) in SI

• The force produced by pressing on something.

• The pressure due to the weight of the atmosphere (air and water vapor) on the earth's surface. The average atmospheric pressure at sea level (for scientific purposes) has been defined at 14.696 pounds per square inch absolute.

• The amount of force acting (pushing) on a unit of area. Usually expressed in pounds per square inch (psi).

• is defined as the force exerted against the Earth's surface by the weight of the air molecules above it. Average atmospheric pressure at sea level is 1013.2 millibars or 29.92 inches of mercury. A drop in atmospheric pressure is often associated with cloudy or rainy weather. Conversely, high pressure is usually associated with fair weather. Variations in pressure lead to the development of winds, which in turn influence our daily weather.

• a constraining force

• the apparently continuous and isotropic force exerted on unit area of any real or imaginary surface because of bombardment by molecules of contiguous fluid.

• The load divided by the area over which it acts.

• Feedwater pressure affects both the quantity and the purity of reverse osmosis product water. Lower feedwater pressure causes lower product flow rate and lower product purity.

• the force that a fluid exerts uniformly in all directions within a vessel, a pipe, a hole in the ground,and so forth.

• Refers to the water pressure in the system. The conversions for the most commonly used units are: 1 bar = 1.02kg/cm 2 = 14.5psi = 100kPa = 0.1Mpa = 10m water head

• A type of stress characterized by uniformity in all directions. As a measurable on a surface, the net force per unit area normal to that surface exerted by molecules rebounding from it. In dynamics, it is that part of the stress tensor that is independent of viscosity and depends only upon the molecular motion appropriate to the local temperature and density. It is the negative of the mean of the three normal stresses.