The force acting per unit area on a surface is called pressure.
Pressure can be exerted on an object by:
1. Another solid object:
A solid object can exert a force on another object when they are in contact. The effect of this force depends on the area of contact between them.
2. A liquid object:
A liquid object can exert a force on a solid object. For example water stored in a bucket exerts pressure on the bottom of the bucket.
3. Air or gaseous object:
Air or any gaseous substance can exert a force on a solid or liquid object. The air around us is constantly exerting a force on every object.
Formula to Calculate Pressure:
You have seen that the force acting per unit area on a surface is called pressure.
This can be expressed as a formula, i.e.
Pressure = Force / Area
Since force is expressed in kgf or newtons and area as square meters, the standard unit of pressure becomes kgf/sq.m. or newtons/sq.m. This unit is also expressed as Pascal named in honor of French scientist Blaise Pascal.
To understand the application of this formula, let us study an example:
A man weighting 60 kg is standing on a wooden plank of 0.5m X 0.5m. Find the pressure exerted by the load on the ground.
Force = 60 kgf = 60 X 9.8 = 588 newtons
Area on which this force acts = 0.5 m X 0.5m = 0.25 m.sq.
Pressure = Force / Area
By putting the values in the formulate,
Pressure = 60 kgf / 0.25 m.sq. = 240 kgf/m.sq.
Pressure = 588/0.25 = 2352 Pa or (N/sq.m)
Pressure in Liquids
Liquids take the shape of the container in which they are stored and therefore exert pressure at the base and on its walls. The pressure on its walls is called lateral pressure.
Properties of Liquid Pressure:
1. Pressure in a liquid increases with depth: As the depth o the liquid level increases, the pressure also increases.
2. Liquid exerts lateral pressure: Liquid exerts pressure not only on the base, but also at the walls of the container.
3. Pressure increases when the liquid is denser: Mercury is used in barometers because it is denser and can support a lower column of 76 cm. Instead of mercury, if we use water, we need a longer tube. This shows that for a fixed length of a tube mercury is able to stand higher pressure than water.
4. At the same depth the pressure in a liquid is the same in all directions: The lateral pressure exerted by any liquid is the same in all directions.
5. Liquids tend to seek their own level: If a liquid is taken in an interconnecting vessel with a number of arms, the level of the liquid in each arm will be equal. This shows liquids always tend to seek the same level.
Transmission of Liquid Pressure:
Take a polythene bag and make a few holes on it. Fill the bag with water and close its mouth with a string. Press it hard with your hands.
Water flows out from these holes in all directions with equal pressure.
When pressure is applied to an enclosed fluid, it is transmitted equally in all diretions through the fluid. This is called Pascal’s Law, This law holds good for both liquids and gases.
Application of Pascal’s Law: We see that Pascal’s law is applied in constructing a number of devices:
- Hydraulic Press
- Hydraulic Brakes
- Hydraulic Freight Elevators and
- Hydraulic Jacks