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Swinburne University of Technology Sarawak Campus

What’s that rascal’s law anyway?

October 17, 2012

By Dr Manas Kumar Haldar

We had just graduated – me with a bachelor’s degree in electrical engineering and my friend with a bachelor’s degree in mechanical engineering. “Universities only teach principles,” said my friend, “I better get industrial experience”. He went for a job in a small industry. I met him several months later. He looked glum. “Everything is fine except my boss”, he said. The boss always told my friend, “You pundits with degrees in engineering know nothing. I know every nut and bolt of the machines here”. “True,” said my friend, “I have to learn from him”.

I met my friend a year later. “You look happy; what have you done?” I asked. This is what he told me: “My company does a routine job of pressing plastics with a primitive hydraulic press. Once, there was a big order from a client, but it required a very different pressure. Try as he could, my boss did not get the right pressure with his “nuts and bolts”. “Sir, using Pascal’s law …” Stop talking, you pundit”, the boss said. “What’s that rascal’s law anyway? Go and fix the problem”. My friend designed the weights to be put on one arm of the machine and he got the required pressure. From that time, the boss stopped using derogatory words.

So what is Pascal’s law? In 1646, French physicist Blaise Pascal observed that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions and produces a force perpendicular to the surface of the fluid. He discovered this law in an experiment using water in a barrel. Liquids and gases are called fluids. An incompressible fluid is one whose volume does not change when pressure is applied. Pressure is force divided by area. Now imagine that you need a certain pressure over a large area. You need a large force. The most primitive hydraulic press solves this problem using Pascal’s law. It consists of a large diameter tube joined with a small diameter tube and filled with a liquid. A smaller diameter, and hence area requires a much smaller force to produce a required pressure. By Pascal’s law the same pressure is transmitted to the tube with a larger area. By putting suitable weights (force) on the smaller diameter tube, my friend managed to get the right pressure in the larger diameter tube.

What is so great about Pascal’s law? Using simple physics, even a school boy can prove it. But remember that in Pascal’s days, the laws of physics were being discovered. Moreover, Pascal was one of the scientists who laid the foundation of a branch of physics called fluid mechanics. The branch of fluid mechanics that deals with unchanging or slowly changing forces in water is called hydrostatics. Hydrostatics is routine study for civil engineers. Engineers who designed the Bakun dam, for example, had to use the principles of hydrostatics. When forces change rapidly in a fluid, as in the air surrounding a flying plane, the equations of fluid mechanics get very complicated and difficult indeed. You can learn them if you are studying at a university like Swinburne, not in an industry. This branch of fluid mechanics is called aerodynamics.

Is Pascal’s law of any use in our daily lives? Notice that the law allows what may be called a force multiplier. Using a small force in a narrow tube, one can produce a large force in a wider tube. There are other types of force multipliers. For example, one can produce a large force with a small force using a lever. However, a lever is a rigid rod which might make its use inconvenient in many applications. A tube containing a liquid can be bent in any way you please and it will still transmit the pressure. This is used in the brake of a car. Let’s suppose you are sitting comfortably in the driver’s seat of a car running at 60 kilometres per hour. To stop the car, you push the brake pedal with your foot. The system has several mechanisms, but ultimately, it produces a pressure in a liquid in a tube. This pressure is then transmitted to the braking shoes at the end of a larger tube. The braking shoes press against the inner rim of the wheels, thereby stopping the car.

Unfortunately, many terms in science have been borrowed to describe other human activities. For example, the military uses the term force multiplier. It essentially means additional sophisticated equipment to kill people more efficiently. But think of the force multiplier that “rascal’s” law provides us. It has much to contribute to the betterment of our lives. But most importantly to me, it brought a smile to my friend’s face. If you study at Swinburne, you will learn many such “impractical” laws of physics. And who knows, some day you might be using such principles for practical work like my friend.

Dr Manas Kumar Haldar is an associate professor with the Faculty of Engineering, Computing and Science. He is contactable at mhaldar@swinburne.edu.my