What is Pressure?

When a force acts on a body, it sometimes becomes necessary to consider not only the force but, also the area on which it acts. For example, a tractor with wide wheels can move over soft ground because its weight is spread over a large area. As a result, the pressure on the ground is less and it does not sink so deeply. On the other hand, a nail can be driven into a wood because; the very high pressure exerted over the small area of the point is more than the wood can stand. The greater the area over which a force acts, the less is the pressure; conversely, the smaller the area, the greater the pressure.

Based on this observation and fact, the pressure is thus defined as the force per unit area or Pressure= force/area.

This is how; we can insert a nail into the wood.

Among the three states of the matter, the most discernible of the effects that pressure has on a matter state, has been noticed in the liquids although, the impact of increasing pressure in gases has been noticed in their being converted into a liquid state and the same has its impact in certain crystalline solids being noticed in their exhibiting a unique phenomenon of piezoelectricity. Yet, the practical application of pressure & its impact has glaringly been noticed in liquids and thus, constitutes a part of our everyday Physics…

Pressure in Liquids:

This is manifested when the weight of a liquid pulls it down into its container, causing a pressure on the container and on any object in the liquid.

Pressure Laws for Liquids: These pertain to the behaviour of a liquid in an open vessel and may be stated as below:

Pressure in a liquid increases with depth: Because, the farther down you go in a liquid, the greater shall be the weight of the liquid above.

Example:  Water spurts out fastest and farthest from the lowest hole. That is why; the dam of a reservoir is being made thicker at the bottom than at the top just because, the water pressure is greater at the bottom.

ii) Pressure at one depth acts equally in all directions:

Example:  The can of water having similar holes all round it and at the same level, water will come out as fast and as far from each and every hole of the can.

iii) A liquid always finds its on level: The pressure at the foot of a liquid column depends only on the vertical depth of the liquid and not on the width or shape of the tube.

iv) Pressure depends on the density of the liquid: The denser the liquid is, the greater shall be its pressure at any given depth of a liquid column.

♥Everyday Physics involved in the use of Pressure:

I) HYDRAULIC MACHINES: Hydraulic machines work by using pressure in liquids. Their action and the principle of their functioning depend on two chief facts about liquids. Firstly, that the liquids are almost incompressible (that is, their volume cannot be reduced by squeezing or compression) and secondly, that the liquids always pass on to all parts of the liquid, if any pressure applied onto them. This means that they distribute the pressure evenly among all the parts of the liquid.

The principle on which hydraulic machines work is used in our practical life. In hydraulic car brakes, when the brake pedal is pushed or pressed down, the piston in the master cylinder exerts a force on the brake fluid that being a liquid distributes the pressure and hence, the resulting pressure is transmitted equally to eight other pistons. Once this is done, these pistons eventually, force the brake pads against the wheels and hence, the car is stopped. Similarly, a hydraulic jack on which cars are mounted in a workshop has a platform on top of a piston and is used in garages to lift cars. A hydraulic press is constructed similarly…

Connecting concepts: Why do we feel/experience “ear popping” in an aircraft?

A one line answer to this question is the effect of atmospheric pressure. Although, the air forming the earth’s atmosphere stretches upwards for hundreds of kilometers, it thins out very rapidly after ten kilometers or so. Like a liquid, it also exerts a pressure in all directions which significantly decreases with height to which what we call as “atmospheric pressure.” At sea level, atmospheric pressure is very large and equals about 100,000 Pa = 100 kPa. (refers to kilopascals). We as individuals, do not normally feel atmospheric pressure because, the pressure inside our bodies is almost the same as that of the outside atmospheric pressure. Our ears however, are sensitive to pressure changes and this is why people experience ear ‘popping’ in an aircraft at take-off. This is due to the outside air pressure falling as the aircraft climbs up so that a pressure difference is created between the air in the middle part of the ear and that in the outer ear as a consequence of which the eardrum becomes distorted. Swallowing helps to equalize the pressures. Modern high-flying aircrafts have pressurized cabins in which the air pressure is increased sufficiently above that of the outside pressure in order to safeguard the crew and passengers from difficulty with breathing.

The large value & practical applications of atmospheric pressure was first demonstrated by von Guericke, who invented the vacuum pump. (Vacuum is a space that has no air). About 1650, he fitted together two large hollow metal hemispheres to give or form an airtight-sphere, which he then evacuated. So good was his pump that it took two teams, each of eight horses, to separate the hemispheres.

Fluid Pressure in Everyday Life:

♥When you use a drinking straw, you suck at the straw to the consequence that your lungs expand the moment you try to suck in and thus, the air passes into them from the straw. Atmospheric pressure pushing down on the surface of the liquid in the bottle is now greater than the pressure of the air in the straw and so forces the liquid or your favorite cola beverage right up into your mouth. (Remember: flow from high to lower pressure).

When a rubber sucker is moistened and pressed on a smooth flat surface, the air is moistened and pressured on a smooth flat surface, the air inside is pushed out. Atmospheric pressure then holds it firmly against the surface. Such rubber Suckers are extensively used by all of us as towel holders at home or in industry for lifting metal sheets etc.

In a vacuum cleaner, the fan creates a partial vacuum in the bag which causes air, carrying dust, to rush through the cleaning attachment into the bag.

In vehicles with ‘power brakes’, atmospheric pressure supplies an extra force to the brakes. In this case, the engine removes air from both sides of a piston in a cylinder which links the braking system to the brake pedal. When the latter is pushed, a valve opens to let air into the right-hand side of the piston and the resulting pressure differences forces the piston to the left.

Syringes of various kinds are used by doctors to give injections and by gardeners to spray plants. A syringe consists of a tight-fitting piston in a barrel, and is filled by putting the nozzle under the liquid and drawing back the piston. This reduces the air pressure in the barrel and atmospheric pressure forces the liquid up into it. Similarly, pushing down the piston drives liquid out of the nozzle.

When the piston of a bicycle pump is pushed in, the air between it and the tyre valve is compressed. This pushes the rim of the plastic cup washer against the wall of the barrel to form an airtight seal. When the pressure of the air between the plastic washer and the valve is greater than the pressure of the air in the tyre, air is forced past the tyre valve into the tyre. When the piston is drawn back, the tyre value is closed by the greater pressure in the tyre. Atmospheric pressure then forces air past the plastic washer (which is no longer pressed hard against the wall) into the barrel.

Pressure and Diving: For every 10.3m (approx. 10m in sea water) that a diver descends down, the pressure in his body increases by one atmosphere. The aqualung diving suit incorporates a rubber helmet fitted with a circular window and is supplied with air from compressed-air cylinders carried on the wearer’s or diver’s back. Using this apparatus, experienced divers can descend for very short periods to a maximum depth of about 60m, where the total pressure is amazingly about seven atmospheres. At depths in the neighbourhood of 45m, they can work for periods of about 15 minutes. It is dangerous to stay longer at these depths, since, as a result of the high pressure, an excess of nitrogen dissolves in the blood and on return to the surface, nitrogen bubbles form in the blood in the same way that bubbles are formed in a bottle of soda water when the cork is removed. Such a condition causes severe pain or even death in a condition so called as ‘decompression sickness’. Noted that the danger to health from the painful ‘diver’s bends’ as this condition is commonly called as is, greatly reduced if a mixture of 8 per cent oxygen and 9.2 per cent helium is used in gas cylinders.

Fluid pressure & Archimedes Principle

The collective term applied to all liquids, gases and materials in the molten state is fluid. A solid body is said to be immersed in a fluid when it is either partially or completely surrounded by and in surface contact with the fluid. On earth everything is immersed in a fluid (air). Displaced Fluid is the fluid which would occupy the volume occupied by the immersed part of a solid body.

Any object in a liquid, whether floating or submerged, is acted on by an upward force of upthrust. This makes it seem to weigh less than it does in air. The upthrust arises because, the liquid pressure, which pushes on all sides of the object, is greatest on the bottom where the liquid is deepest. Experiments that were conducted with other liquids and also with gases finally led to the formulation of a general principle what we call as Archimedes principle which states that: “When a body is wholly or partially submerged in a fluid, the upthrust equals the weight of the fluid displaced.”

The upthrust thus, depends on the volume of the object and not on its weight.

Why do objects sink and float?

Physics behind the FLOATING & SINKING of objects:

Principle or laws of Flotation: A stone held below the surface of water sinks when released: a cork rises, however. Why? This is because, the weight of the stone is greater than the upthrust (also buoyancy or buoyant force) on it (that is, the weight of water displaced) and consequently, there is a net or resultant downward force on it. If the cork has the same volume as the stone, it will displace the same weight (and volume) of water. The upthrust on it when completely immersed is therefore the same as for the stone, but it is greater than the weight of the cork. The resultant upward force on the cork makes it rise through the water. When an object such as a wooden block floats in water, the upthrust equals the weight of the object. The net force on the object is zero, and the weight of water displaced equals the weight of the object in air. This is an example of the principle of flotation. A floating object always displaces its own weight of fluid.

♥Why does an object sink? An object sinks in any liquid that has a smaller density than its own: in other liquids it floats, partly or wholly submerged. For example, a piece of glass of relative density 2.5 (density 2.5 g/cm³) sinks in water (density 1.0 g/cm³). An iron nail sinks in water, but an iron ship floats because its average density is less than that of water.

The direction of buoyancy (or buoyant force) is always vertically upward. It acts at the centre of buoyancy (i.e. at the centre of gravity of the volume of displaced fluid). The weight of the immersed body acts downward, and at the centre of gravity of the solid.

Connecting concepts: How do Ships Float?  The material (mostly iron) of which the ship is built is much denser than water, but the ship is built in the form of a large shell or ‘hull’ as it is called. This shell displaces a very large volume of water, and so the ship floats. However, if the ship is loaded to such an extent that its total weight is more than the weight of the water which the whole ship can displace it will sink. The ‘Plimsoll Line’ on the hull of all sea-going vessels is drawn to shop the depths to which the ship can safely be loaded in different parts of the world and ship can safely be loaded in different parts of the world and in different seasons. The winter load lines are lower than the summer ones as sea water is denser in winter (provided the temperature is above 4⁰C) than summer. A ship will float lower in fresh water than in salt water because fresh water is less dense than salt water. 

How do Submarines able to move about under water? Submarines are able to move about under the surface of the sea water because; their average density can be controlled. A submarine sinks by taking water into its buoyancy tanks. Once submerged, the upthrust is unchanged but the weight of the submarine increases with the inflow of water and it sinks faster. To surface, compressed air is used to blow water out of the tanks.

Balloons and Airships:  A balloon with hot air of hydrogen weighs less than the weight of cold air it displaces. The upthrust is therefore greater than its weight and the resultant upward force on the balloon causes it to rise. A hot air balloon carries a gas burner beneath it; a quick blast on the burner about every 30 seconds keeps the air inside the balloon hot. When balloons are fitted with motor-driven propellers and steering devices, they are known as airships.

Archimedes law of buoyancy for liquid is equally valid for gases. A large, hollow body such as a balloon can displace more than its own weight of air, and so can float in air. Since the air is less dense higher up, a balloon will rise only to the level where the weight of the displaced air becomes equal to its own weight.

(It may be noted that due to its inflammability, hydrogen has been replaced by helium in balloons and airships.)

FLUIDS IN MOTION: BERNOULLI’S PRINCIPLE: 

The pressure is the same at all points on the same level in a fluid that is at rest; but this is not so when it is moving. When a fluid moves even with moderate speed, important new forces come into play. The most evident effect is the resistance that air, for instance, offers to the movement of objects through it. The actual resistance force increases with cross-section area of the moving body and especially with its speed of motion. In addition, the shape of the object is of great importance.

A liquid will only flow through a pipe if the pressure at one end of the pipe is higher than that at the other, that is, there is a pressure difference between the ends of the pipe. The pressure at different points in a liquid flowing through (a) a uniform pipe and (b) a pipe with a narrow part is shown by the height of the liquid in the vertical manometers in the figure below:

In (a) the pressure drop along the tube is steady. In (b) the pressure falls in the narrow part B but, rises again in the wider part C. Since in a certain time the same volume of liquid passes through B as it enters A, the liquid must be moving faster in B than in A; the pressure in the liquid thus decreases as the speed of the liquid increases. Conversely an increase of pressure accompanies a fall in speed. This effect, called Bernoulli’s Principle, is stated as follows: “when the speed of a fluid increases, the pressure in the fluid decreases and vice versa.”

Bernoulli effects are to be seen in air streams also. The pressure falls in the moving air stream. Two sheets of paper come together when you blow between them and a paper rises when you blow over it.

Everyday Physics of Bernoulli’s principle: This may be described in other words, as the applications of Bernoulli’s principle in our common day observations.

♥As a fluid comes out of a jet, its speed increases and its pressure decreases. This fact is used in a Bunsen burner in which air is drawn into a carburetor by a jet of petrol in a similar manner.

♥A spinning ball takes a curved path because, it drags air round with it thereby, increasing the speed of the air flow on one side and reducing it on the other.

♥An aircraft, wing, called an aerofoil, is so shaped that the air has to travel farther and so faster over the top surface than underneath. The resultant upward force on the wing provides ‘lift’ for the aircraft. Contrary to what one might expect, the front of the body is broader than the rear. (But if the body is to be a high-speed jet plane or rocket traveling faster than sound, a sharp-nosed shape gives best performance).

♥In the helicopter, the airflow over the surface is produced by whirling the rotating fans, rather than by rapid motion of the whole plane through the air. As a result, a helicopter can hover over one spot on the ground, or even move in a backward direction.

Notwithstanding these all, a number of other familiar observations and devices can be described in terms of Bernoulli’s law or principle, say for example:

? Two cars, passing each other at high speed are in danger of colliding sideways because of the decrease in air pressure in the space between them.

?A strong gale is capable of lifting the roof off a house in the same way.

? In an atomizer (sprayer), a stream of air is blown across the end of a small tube that dips into the liquid. The decreased pressure at the side of this air stream allows normal air pressure, acting on the surface of the liquid in the bottle, to push the liquid up the tube. There the moving air breaks it up into small drops and drives it forward.

The “Energy” and its different manifestations

As we already know that the nature is entirely made up of the matter what we may broadly classify as living and non living matter. There is a great unifying principle of the nature that at the hierarchical structural organization of the matter, there is hardly any difference between the living and the non-living matter at least, at the subatomic level. The same is true about the energy transformations that the matter constantly undergoes in nature such that we come to witness a whole lot of energy transformations around, incessantly been taking place to the effect that the total energy content of the universe has always remained constant, irrespective of the fact that during each of such energy transformations, some amount of energy does go waste to the environment in the form of heat. But then, this is how that the matter-energy complex of the nature has been maintained ever since the formation of earth and the evolution of the first life has happened on it…

Going by the above inherent truth of the nature, it is not hard to conceive even for a layman that what all we study under Physics as various phenomena and properties of the physical matter; being manifested either as heat, light, electricity or magnetism etc. are, but the consequence of the changes in the matter and the transformations of the energy that occur incessantly in the matter-energy complex of the nature. And it is the sheer ingenious nature of the human mind that this matter-energy complex of the nature has been innovatively exploited by the man to his advantage and given us something of the nature of everyday PHYSICS.

Before do we go about discussing the various manifestations of energy, it sounds rather pertinent to know what energy is all about in its simplest terms: