pressure
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In a fluid, the force exerted normally (at right angles) on the surface of a body immersed in the fluid. The SI unit of pressure is the pascal (Pa), equal to a pressure of one newton per square metre. In the atmosphere, the pressure declines with increasing height from about 100 kPa at sea level to zero where the atmosphere fades into space. Pressure is commonly measured with a barometer, manometer, or Bourdon gauge. Other common units of pressure are the bar (100 kPa) and the torr (approximately 133 Pa).
Pressure can be worked out using the formula: pressure = force/area. For example, farm tractors have wide tyres with relatively large areas in contact with the ground. This means that the pressure exerted by these areas is less (the force, or weight, is spread out over a larger area), which helps prevent the tractor wheels from sinking into the ground. Similarly, the head of a safety pin pressed down onto a table will not penetrate into the table; the force exerted by the head is spread out over a relatively large area and the pressure is lower. On the point of the pin, the force is spread over a much smaller area, the pressure is consequently greater, and the point will penetrate into the table.
Absolute pressure is measured in relation to the zero pressure of a vacuum; gauge pressure is the difference between the absolute pressure and the local atmospheric pressure. In a liquid, the pressure at a depth h is given by ρgh where ρ is the density and g is the acceleration of free fall.
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