Concept of pressure

What is Pressure?

Have you ever noticed that your ears get blocked and hearing becomes lower when you ascend or descend a mountain at a fast pace? Or why do our knives need to be extremely sharp? The answer lies in the concept of pressure.

If you try to hammer a bowling pin into a wall with all your might, nothing would happen. However, if you hammer a nail with the same force, the nail has a high probability of penetrating the wall. This depicts that most times only a large magnitude of force isn’t enough, but also over what surface area the force is distributed matters as well. Now if we take the example of two different nails, one sharp and one blunt, the force required for the sharp nail to penetrate the wall will be much lesser than that for the blunt nail. This clearly shows that the area of concentration of the force plays a vital role in pressure distribution.

Define Pressure

The force applied perpendicular to a unit area over which the force gets distributed is defined as Pressure. It is the ratio of the force applied over the area over which the force is being applied.

According to the definition, when force ‘F’ Newton is applied perpendicularly over a surface area ‘A’, then the pressure is defined as the ratio of the force to the area i.e F to A. Therefore, the formula of pressure (P) is :

P = F / A.

Units of Pressure

There are various units that can be used to depict pressure, but the SI unit of pressure is Pascal (Pa). A pascal is defined as a force of 1 Newton applied over a 1m2 or 1-meter square area. It is also measured in units such as atm or equivalent of atmospheric pressure and over all measured in comparison to 760mm of mercury which also is considered as 1 atmospheric or 1atm of pressure. It can be expressed in several different systems of units such as millimetres (or inches) of mercury, pounds per square inch (psi), dynes per square centimetre, millibars (mb), standard atmospheres, or kilopascals.

Atmospheric Pressure

The earth is encapsulated by a layer of gases that exert a force or in simpler terms a pressure on the surface of the earth known as atmospheric pressure. The value at sea level is 101325 Pa. It can be easily measured with a mercury column or mercury barometer and hence atmospheric pressure is also known as barometric pressure.

Factors Affecting Pressure

Pressure is inversely proportional to the surface area the force is being applied over, thereby the pressure can be increased or decreased without any change in the force applied. With force remaining constant, the pressure can be increased or decreased by decreasing or increasing its surface area respectively.

In simpler terms if the area is smaller the pressure is high while if the area is larger the pressure is low. It is for this reason that the knife edge is sharp where the knife distributes the force over the cutting edge that has a lower surface area. As the knife gets blunt, the area increases and thereby the pressure reduces, increasing our effort.

For a similar reason a karate chop expertly delivered will have a much deadlier effect than an open palm slap. The force applied being the same, the surface area of the open palm is larger than that of the karate chop which leads to the distribution of the force over a larger area and reducing the effect of the attack.

Not always, would you want a higher pressure and hence in a lot of situations a larger surface area is preferred such as in skiing and surfing. The ski & surfing boards increase the overall area on which our body weight acts thereby allowing us to glide and float over the surface of water or ice extremely easily. Similarly, a board pin has one side flat and one side sharp enabling us to press the pin into the board with ease without too much pressure on our fingers.

Conclusion

Pressure is defined as the force applied per unit area. The SI unit of Force is Pascal or Pa. It is also measured in atm, mm of Hg, PSI etc.
Pressure is inversely proportional to the surface area over which the force is being applied. Increase in surface area reduces pressure while decrease in surface area increases pressure assuming that the force applied is constant.