Fluids

Thrust and pressure

Earth is surrounded by a lot of gases. This envelope of gases around our planet is called atmosphere. Atmosphere is vital for the survival of all life forms on our planet. As gases have mass, hence, they exert pressure on their surroundings. Atmosphere consists of gases. Hence, it also exerts pressure on the earth’s surface along with all the life forms living on it. This pressure exerted by the atmosphere is called atmospheric pressure.

We can now define atmospheric pressure as:

The force exerted on a unit area by a column of air above the earth’s surface is called atmospheric pressure.

The value of atmospheric pressure in the SI system is 100000 N/m² or 100000 Pa.

Variation in Atmospheric Pressure

Atmospheric pressure varies with:

• Height — With increase in height from the sea level, atmospheric pressure decreases. Following two factors are mainly responsible for this decrease in atmospheric pressure with height:
  (1) decrease in height of air column results in a linear decrease in the atmospheric pressure and
  (2) decrease in density of air with height results in a non-linear decrease in atmospheric pressure
  
  

• Season— With change in season on earth, the water vapour content in the atmosphere also changes. Therefore, the variation of pressure occurs with season change.

• Temperature— With increasing temperature, the atmospheric density decreases. Therefore, atmospheric pressure decreases with increasing temperature.

Effects of Atmospheric Pressure

• It is interesting to know that there is such big pressure acting all around us. Have you ever thought that how this pressure is not felt by us? It is not felt by us because our blood also exerts pressure on our body from inside. This pressure of blood balances the pressure of the atmosphere such that the atmospheric pressure is not felt by us.

• When we travel in an aeroplane, our nose may start bleeding, if the aeroplane is not pressurised properly. This happens because the atmospheric pressure at high altitude is lesser than the blood pressure inside our body. It is this difference of pressure that bursts our capillaries within our nose, making our nose bleed. It is for the same reasons an aircraft should be pressurised properly such that the pressure inside the aircraft remains the same as the normal atmospheric pressure at the ground level. Even astronauts wear space suits to counter the zero pressure that exists in the outer space. Fishes in deep sea water experience more pressure than we feel at land. Hence, their internal body pressure is more than ours. If these fishes are brought out of the water, then their body will burst because of the excess outward pressure of their internal body fluid.

• If you take a fountain pen to higher altitude, then you will see that the pen starts leaking. This happens because of low atmospheric pressure at high altitude. The ink pressure inside the pen becomes higher that the outside atmospheric pressure and as a result, the pen starts leaking.

• Sucking from a straw works on the same principle. When you suck the air out of the straw, pressure falls inside the straw. This fall in pressure is compensated by the liquid that is forced up by the atmospheric pressure.

Fluids have a very interesting property. They can transmit pressure applied on them throughout the fluid in every direction. French mathematician Blaise Pascal conducted several experiments on transmission of pressure through liquids and reached the conclusion that:

If a pressure is applied to a confined incompressible fluid at any point, then the pressure gets transmitted equally to all parts of the fluid in all directions and it always acts at right angles to the surface of the containing vessel.

This is known as Pascal’s law or the principle of transmission of fluid pressure.

If you apply pressure from top on water kept in a pot, then the pot will not break from its bottom, but from the sides. Can you explain why this happens?

Let us perform an experiment to find out the reason.

For the experiment, you require a vessel with four openings of equal cross-section. The openings are fitted with four water-tight movable pistons P, Q, R, S. Now, fill the vessel completely with water through one opening, keeping the other openings closed. If you push piston P with a force of 1 kgf, then you will find that the three other pistons also move and you have to apply 1 kgf force to each piston to stop their motions. This shows that the applied force on P is equally distributed to all the pistons. Now, if you increase the area of one of the pistons (say S) three times, then what will happen?

You will observe that on increasing the cross-sectional area, the force …