THE CHINGCHOK

Under pressure

You against the world

DAVID CANAVAN

Every second of every day, you are being bombarded by air particles that may be travelling at over 1,000 kilometres per hour, and which collectively could weigh many tonnes! Thankfully, you don't realise any of this as you are at equal pressure with the air, but if you weren't, the consequences would be devastating.

We inadvertently use, account for and apply the science of physics every day in our lives. Rarely a day goes by when I don't have to stop a pupil sitting with two legs of his or her chair in the air because the two other chair legs, as a result of pressure, scuffs the lab floor. Pressure is everywhere, and hopefully, after reading this article, you will understand the science behind it.

Classic example

If you had to choose what stood on your foot, either the foot of an elephant or the heel of a woman in high heels, which would you choose? Obviously, neither would be a great result, but understanding the pressures involved may sway your decision.

This is a classic example of pressure in the world of physics because it bases the answer on the formula for pressure, which is worked out by calculating the force divided by the area. The force, as with all forces, is measured in Newtons (N), and the area is measured in metres squared, which gives a pressure value of Newtons per metre squared, or N/m2.

This value is also known as 1 Pascal (Pa) which is the international standard unit (SI) for pressure. It is named after Blaise Pascal, a French scientist and mathematician. A Pascal, by definition, is the force of one Newton (98.1 grammes) acting downwards on one square metre.

If you take into account that an elephant will have a mass of about 4,000kg, this converts to a weight of approximately 40,000 Newtons. The weight on each foot, therefore, will be about 10,000N. The base of one of its feet will have an area of about 0.07m2; so, using the pressure formula, you can work out that the pressure exerted by an elephant's foot is approximately 143,000Pa.

When comparing with a woman in high heels, we can afford to be moderate. Let's take an average Thai woman, who has a mass of 50kg. This means she weighs about 500N, and so there is 250N of weight for each foot. Her high heel shoe will have an area of roughly 0.0001m2. Therefore, 250 0.0001 equals 2,500,000Pa of pressure - over 17 times more than that exerted by the elephant! I know which I would choose!

Simply physics

The above example may seem obvious, and that is the beauty of much of physics. It simply explains the applications of many of our daily activities in a logical, mathematical and quantifiable way. Having an understanding of this simply allows you to understand the world a little better.

Take the drawing pin. You obviously know to push the flattened side with the point into the wall. But hopefully, you can appreciate why you do that when you think of the force you apply over the relatively large area. This transfers as the same force over a tiny area (the pin head) which creates a lot of pressure and forces it into the wall. Ingenious!

Pressure in liquids

Pressure is transmitted evenly through liquids in all directions. The particles in liquids are very close to each other, but they are not held in a fixed state as with solids. This means that if pressure is exerted on a liquid, it will pass from one particle to another quickly and uniformly, meaning it is transferred evenly.

This knowledge of pressure in liquids has been applied to many things, but one of the most useful is in the science of hydraulics. Hydraulics uses pressure in liquids to magnify a force, an application of which is used in the brakes of vehicles.

As previously stated, pressure is worked out by taking the force divided by the area. In hydraulics, you have a small piston known as the master piston. In a car, this is your brake pedal, to which you apply pressure when you want to slow down or stop.

What this does is transmit the pressure evenly through the liquid to the four disc brakes by the wheels, which also have pistons (known as slave pistons). The pressure is the same from where you pressed the brake pedal, to where it is transmitted to the brake pistons.

If the area of the master piston is small, your pressure multiplied by the area will give you a certain amount of force. When this same pressure is applied to the pistons in the brake discs, which have a much greater area, the force produced, therefore, must be greater.

Pressure multiplied by area gives you the force, meaning that if the pressure is the same, but the area increases, the force applied to the slave pistons must increase. This creates enough force for you to stop a two-tonne vehicle with just your foot! Isn't pressure clever?

Pressure increases with depth

As all divers know, pressure increases with depth. This is true for any fluid, while bearing in mind that gases are also classed as fluids. If you see any dams, for which Thailand is renowned for, such as the Klong Tha Dan dam in Nakhon Nayok province, you will notice that they are built gradually thicker as they get deeper. This is because the pressure of the water is greater the deeper it gets. You may also notice the pressure hurting your ear drums as you dive deeper until the pressures are equalised.

Humans have only once been to the deepest part of the ocean, the Mariana trench in the Pacific ocean. The pressure at 11km deep would be equivalent to about 37 jumbo jets on top of you, so it is no easy task to simply go there again!

Pressure also increases with depth in our atmosphere. At sea level, standard atmospheric pressure is measured using the liquid metal mercury in a barometer, and is 760mm/Hg (millimetres of mercury). Weather affects air pressure, but if you go down a coal mine, pressure will increase, whereas if you go up a mountain, the pressure will decrease.

If you go up the highest mountain, Mount Everest, the pressure decreases so much that water boils at a much lower temperature.

As you are at less depth in the atmosphere, and therefore do not have all of the enormous weight of all the atmosphere upon you, pressure is decreased so much that water boils at 60C as opposed to 100C at sea level!

Referring back to my pupils who tilt their chairs back, I threaten them by saying that, knowing their weight and the area of the base of the two legs on a chair, I will set them the task of finding out the pressure they are exerting on my floor. They all decline, of course, and sit properly, but in reality, it is not that complicated, and it is actually quite interesting!

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! You may contact Dave at davidc@gardenbangkok.com .

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