Revise: Pressure
Pressure from force and area, pressure in fluids, density, relative density (specific gravity), and how a hydrometer measures it.
P = F/A. A smaller area concentrates the same force into a bigger pressure.
200N over 0.002m² → 100,000 Pa.
RD = ρ(substance)/ρ(water). No units.
800 kg/m³ → RD = 0.8, floats.
Floats higher in a denser liquid — reads relative density directly.
RD 1.03 in seawater vs. 1.00 in fresh water.
Pressure (Pa) ≠ force (N) — a tiny force on a tiny area gives huge pressure.
A drawing pin: 5N on 1mm² → 5,000,000 Pa.
Fluid pressure depends only on depth, density, and g — not container shape.
1.5m of water → 14,700 Pa.
Every pure substance has a characteristic density — used to identify materials.
2.7 g/cm³ × 40cm³ → 108 g.
RD > 1 sinks; RD < 1 floats. Used to test battery acid, milk, etc.
2700 kg/m³ → RD = 2.7.
A floating object displaces liquid equal to its own weight — less volume needed in denser liquid.
Brewers track fermentation via dropping hydrometer readings.