Pressure in Fluids, Upthrust & Archimedes’ Principle — A/L Physics

What must I remember about Pressure in Fluids, Upthrust & Archimedes’ Principle?

• Hydrostatic Pressure Equation: Fluid at rest exerts normal pressure equally in all directions; Pressure at depth h below open surface: P = P_atm + h ρ g; Gauge Pressure P_gauge = h ρ g.
• Archimedes' Principle Statement: When a body is wholly or partially immersed in a static fluid, it experiences an upward buoyant force (Upthrust) equal to the weight of the fluid displaced by the immersed volume: Upthrust U = V_submerged * ρ_fluid * g.
• Law of Flotation: A floating body displaces a volume of fluid whose weight is strictly equal to the total weight of the floating body: W_body = U => V_total * ρ_body * g = V_submerged * ρ_fluid * g.
• Fraction of Floating Body Immersed: V_submerged / V_total = ρ_body / ρ_fluid; for ice (ρ = 917 kg/m³) in water (ρ = 1000 kg/m³), approximately 91.7% of volume is submerged below water surface.

Test yourself (1 minute)

1. Center of Buoyancy (B)

   The point through which the resultant upward buoyant force acts, coinciding with the center of gravity of the displaced fluid.

2. Apparent Loss of Weight

   When submerged in fluid: Apparent Weight W_app = True Weight W - Upthrust U = V (ρ_solid - ρ_fluid) g.

3. Hydrometer Operating Principle

   Constant-weight variable-immersion hydrometer sinks until displaced liquid weight equals hydrometer weight; stem calibration scale is non-linear (divisions spaced farther apart near top / lower density).

Practise Pressure in Fluids, Upthrust & Archimedes’ Principle questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 10: Mechanical Properties of Matter

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Unit 10.5 · Year 2 · Term 3