Physics 4 — Thermo & Fluids

Fluids at rest — pressure & buoyancy

continues from lesson 3 — values defined earlier in the course stay live here

ELI5: in a fluid, depth is weight: the column above you presses down with P = ρ·g·h, so every ~10 m of water piles on another atmosphere. And Archimedes says the fluid you shove aside pushes back with its own weight — that's buoyancy.

\rho _{w} := 1000 kg/m^3

= 1000 kg / m^3 water density

g := 9.81 m/s^2

= 9.81 m / s^2

P_{30m} := \rho _{w} * g * 30 m

= 294300 Pa gauge pressure at 30 m — about 3 atm

m_{sw} := 70 kg

= 70 kg

V_{disp} := 0.068 m^3

= 0.068 m^3 water displaced by a swimmer, lungs full

F_{buoy} := \rho _{w} * g * V_{disp}

= 667.08 N buoyant force

✓ pass

F_{buoy} < m_{sw} * g

lungs full ≈ nearly neutral: this swimmer sinks slowly (try 0.072 m³)

Real-world hook: pressure-with-depth is why deep divers need decompression and why submarine hulls are thick; buoyancy floats steel ships (hollow → lots of displaced water) and runs a hot-air balloon.