What is Pressure?
“The force acting normally per unit area on the surface of a body is called pressure”.
Press a pencil from its ends between the palms. The palm pressing the tip feels much more pain than the palm pressing its blunt end. We can push a drawing pin into a wooden board by pressing it by our thumb. It is because the force we apply the drawing pin is confined just at a very small area under its sharp tip. A drawing pin with a blunt tip would be very difficult to push into the board due to the large area of its tip. In these examples, we find that the effectiveness of a small force is increased if the effective area of the force is reduced. The area of the tip of pencil or that of the nail is very small and hence increases the effectiveness of the force. The quantity that depends upon the force and increases with decrease in the area on which force is acting is called pressure. Thus pressure is defined as:
Thus Pressure P=F/A
Pressure is a scalar quantity. In SI units, the unit of pressure is Nm-2 also called pascal (Pa). Thus:
1N m-2 = 1 Pa
Pressure in Liquids
Liquids exert pressure. The pressure of a liquid acts in all directions. If we take a pressure sensor (a device that measures pressure) inside a liquid of the liquid varies with the depth of sensor.
Liquid pressure formula
Consider a surface of area A in a liquid at a depth h by shaded region. The length of the cylinder of liquid over this surface will be h. The force acting on this surface will be the height w of the liquid above this surface. If ρ is the density of the liquid and m is mass of liquid above the surface, then:
This equation of liquid preesure gives the pressure at the depth h in a liquid of a density ρ. It shows that its pressure in a liquid increases with depth.