December 28, 2015




The pressure of a liquid is decreased if the velocity with which it is moving is increased. Why is this so? This is because a flowing liquid (or even a gas) is made up of molecules. When these molecules hit the surface it is a pressure. When the molecules are going fast, the number of hits are less and hence the pressure is less.

This principle is used to make airplanes fly. What we do is we make the wings of the airplane in such a way that air needs to travel a longer distance on the top surface when compared to the bottom surface. Since the air is moving smoothly on both the surfaces, it takes a longer time for the air moving on the top surface to cover the wing as compare to the air that is moving along the bottom surface.

This means that the velocity of the air moving on the top surface is less when compare to the velocity of air moving along the bottom surface. Hence the pressure on the top surface is lesser when compared to the pressure on the bottom surface. So there is a net upward pressure on the bottom surface and airplane flies! (see figure)


2.1 What is a Gas ?

A gas is a substance that can compressed easily. Gases fill the container in which they are put. Actually the atoms and molecules in a gas are free to move about. There are spaces in between molecules ! That’s why it is a gas ! There are three fundamental variables that define the state of a gas. They are pressure, temperature and volume. These three variables are connected deeply in the form of laws. The first law is Boyles law. This gives the connection between pressure and volume. Pressure is the force per unit area on the wall of container. Volume is the space occupied by the gas. How are these two related and why ?

2.2 Gas Laws

Boyle’s law

If you increase the volume keeping temperature constant, there will be less molecules per unit volume. Hence the hitting of the molecules of the container will be less. Hence the volume is inversely proportional to pressure. This is called Boyle’s law. The next law is Charle’s law. Charle’s law states that the volume of a gas is directly proportional to its temperature provided the pressure is held constant. It is because if you increase the temperature and pressure is kept constant, then the body will increase in volume due to increased movement of molecules.

These are the two fundamental laws that govern all gases.