Introduction to Underwater Physics for Divers

Introduction to Underwater Physics

• This presentation covers the physics fundamental to freediving
• It focuses on the concepts for divers, with specific reference to the Lifras S2 certification.

Fundamental Quantities

• Mass (M): A measure of matter.
  • Unit: kilogram (kg)
• Force (F): A push or pull, causing movement.
  • Unit: Newton (N)
• Weight (P): Force of gravity on an object.
  • Unit: Newton (N)
  • Relationship: Weight = mass × 9.81 m/s² (approximately 10 m/s²)

Confusion: Mass and Weight

• Common mistake: conflating mass and weight
• Weight is a force, while mass is a measure of matter

Capacity and Volume

• Capacity: The amount a container can hold.
• Volume (dm³ & m³): Space occupied.
  • 1 dm³ = 1 litre
  • 1 m³ = 1000 litres

Density (p)

• Mass density (kg/m³): Mass per unit volume
• Density (solids & liquids): Ratio of a substance's mass to the mass of an equal volume of water at 4°C.
• Density (gases): Ratio of a gas's mass to the mass of an equal volume of air (under the same temperature and pressure conditions).

Pressure (P)

• Definition: Force applied over a surface area.
  • Formula: Pressure = Force/Area (P = F/S)
  • Unit: Pascal (Pa) 
  • 1 bar = 100 000 Pa

Atmospheric Pressure (P

atm )

• Definition: Pressure exerted by the atmosphere.
• Value (sea level): Approximately 1 atm = 1013 hPa = 1.013 bar = 760 mm Hg.
• Variation: Decreases with altitude.

Hydrostatic Pressure (P

hyd )

• Definition: Pressure exerted by a fluid (water) column.
• Variation: Increases with depth (due to fluid weight).
  • Approximately 1 bar for every 10 meters of depth

Relative Pressure

• Definition: Pressure measured relative to a reference value (usually atmospheric pressure).

Absolute Pressure (P

abs )

• Definition: Total pressure, encompassing atmospheric pressure and hydrostatic pressure.
  • Formula: Absolute pressure = Atmospheric pressure + Hydrostatic pressure (P abs = P atm

• P hyd )

Air Composition

• Air: Mixture of gases.
  • Primarily nitrogen (N2) and oxygen (O2).

Water Properties

• Water Density: Approximately 1000 kg/m³ (fresh water at 4°C)
  • Approximate 1033 kg/m³ (salt water)
• Thermal Conductivity: Water's thermal conductivity significantly exceeds air's, which significantly impacts temperature changes.
  • Hence the need for necessary diving gear like wetsuits to maintain warmth

Thermocline and Halocline

• Thermocline: Zone of temperature transition between warm surface water and cooler deeper water.
• Halocline: Zone of salinity transition, impacting the density of seawater.
• Impact of both zones: Can influence environmental factors like visibility and the distribution of marine life

Sound Propagation

• Sound in water and Air: Sound travels much faster in water than in air.
• Directionality of sound: Precise direction determination of sound in water is difficult

Light Phenomena

• Light Travel: Light travels slower in denser mediums.
• Four Phenomena in Water: Reflection, refraction, diffusion and absorption are notable for how light interacts with water.
  • Reflection: light "bounces" off a surface.
  • Refraction: light bends when passing through different mediums (like water and air).
  • Diffusion: light scattering from particles.
  • Absorption: light energy converted into other forms of energy (like heat)

Consequences of Refraction in Diving

• Distances and Sizes: objects appear closer and larger than they are when viewed under water.
  • Apparent distance is about 3/4 of the actual distance
  • Apparent size is about 4/3 of actual size

Consequences of Diffusion in Diving

• Visibility: Reduces visibility in water
• Risk/Safety Concerns: Can cause difficulties in evaluating depth

Absorption in Diving

• Light Penetration: Light intensity decreases with water depth.
• Colour Spectrum Penetration: certain colours absorb faster relative to other wavelengths of colour.

Boyle-Mariotte's Law

• Definition: Relationship between pressure and volume of a gas at constant temperature — Inversely proportional (constant temperature)

Dalton's Law

• Definition: Total pressure of a gas mixture equals the sum of the partial pressures of each individual gas.
• Impact on Divers: Applies to the gases in a diver's body.

Henry's Law

• Definition: Dissolved gas amount is directly proportional to the partial pressure of that gas above the liquid, at constant temperature

Applications of Henry's Law

• Diving Accidents: relevant for predicting potential decompression sickness
• Diving Computers: used for calculating effective saturation and decompression schedules