Revision: Fluids >> Fluids Physics (English Medium) ICSE Class 9 CISCE

The force which produces compression is called thrust. Its S.I unit is the newton.

The mass per unit volume of a substance is called density: ρ = M/V

• SI Unit: kg/m³, Dimensions: [ML⁻³T⁰] 

The weight per unit volume of a substance is called specific weight or weight density: W = mg/V

• SI Unit: N/m³

The ratio of the density of a given fluid to the density of pure water at 4°C is called relative density. It is a unitless quantity.

The thrust (Normal Force) exerted by a liquid at rest on unit area of the surface in contact is called pressure.

P = \[\frac {F_⊥_​​}{A}\]

• SI Unit: pascal (Pa) = 1 Nm⁻²
  Dimensions: [ML⁻¹T⁻²]

The difference between the hydrostatic pressure (P) and the atmospheric pressure (P₀), measured by a manometer, is called gauge pressure: P − P₀ = ρgh

The pressure exerted by the atmosphere on the earth's surface is called atmospheric pressure.

The total pressure exerted by a fluid, which includes both the atmospheric pressure as well as any other additional pressure due to the fluid itself, is called absolute pressure.

The difference between the absolute pressure and the atmospheric pressure at a point in a liquid is called gauge pressure.

The phenomenon in which the liquid pressure at a point is independent of the quantity of liquid and depends only upon the depth of the point below the liquid surface is called hydrostatic paradox.

The gaseous envelope surrounding the earth is called the earth's atmosphere.

SI unit of pressure is the pascal (Pa) or Nm⁻²
One Pascal: When a force of one newton acts normally on an area of one square metre (1 m²) then the pressure acting on the surface acting on the surface is called one Pascal.

The pressure exerted by this mercury column is considered as the pressure of magnitude ‘one atmosphere’ (1 atm).

High pressure is an area of the atmosphere where the barometric pressure is higher than its surrounding areas. In this case, the wind from the center of high pressure blows towards the surrounding low-pressure areas.

A low-pressure area is an area in the atmosphere where the pressure is lower than its surrounding areas. In this situation, the wind from the surroundings blows towards the center of low pressure.

The force exerted by the air column on unit cross-sectional area at sea level (= 1.01 × 10⁵ Pa = 1.01 bar) is called atmospheric pressure.

1 atm = 1.01 × 10⁵ Pa = 1.01 bar = 760 torr

P_gauge = P_absolute​ − P_{atmospheric​}

Statement: Pascal's Law states that when pressure is applied to a confined (enclosed) fluid, it is transmitted undiminished and equally in all directions throughout the fluid and to the walls of its container.

Mathematical Expression:

Key Points:

• Pressure changes by the same value at every point inside an incompressible, confined liquid.
• Used in hydraulic machines where a small force on a small area produces a large force on a large area.
• Applications: Hydraulic lift, hydraulic brake, hydraulic press, hydraulic jack.

• Pressure exerted by a liquid column depends on height and density of the liquid column.
• It is independent of the shape of the containing vessel or total mass of the liquid.
• Atmospheric pressure is maximum at the surface of the earth and decreases as we move up into the atmosphere.

• Hydraulic Press — Two cylinders (C & D) filled with liquid; small force applied on piston P₁ (smaller area A₁) is converted into a very large upward force on piston P₂ (larger area A₂), since A₂ > A₁.
• Hydraulic Lift — Works on Pascal's Law to lift or support heavy objects such as cars and trucks using liquid pressure.
• Hydraulic Brakes — Small force on the brake pedal is instantly transmitted equally through brake fluid to all cylinders, producing a large thrust on the wheels to stop the vehicle.