Characteristics of Fluids and Solids
High-Yield Summary
- Solids hold a fixed shape/volume; fluids (liquids + gases) conform to their container. Liquids are incompressible with fixed volume; gases are compressible.
- Density: ρ = m/V. Water's density (1 g/cm³ = 1000 kg/m³) is worth memorizing outright.
- Specific gravity (SG = ρ/1 g/cm³) is dimensionless — compares a substance's density to water's. SG < 1 floats, SG > 1 sinks.
- Pressure is force per unit area (Pa = N/m²) and is a scalar, despite being derived from force — a fluid pushes equally in all directions at a given depth.
- Absolute pressure = surface pressure + fluid contribution (P = P0 + ρgz). Gauge pressure is measured relative to atmospheric (Pgauge = P − Patm).
Density, Weight, and Specific Gravity
ρ = m/V | Fg = ρVg | SG = ρ / (1 g/cm³)
- ρ = Density
- m, V = Mass and volume
- Fg = Weight (gravitational force) of the fluid
- g = Acceleration due to gravity (≈ 9.8 m/s²)
- SG = Specific gravity — dimensionless, compares ρ to water's
- Water's density: 1 g/cm³ = 1000 kg/m³ — memorize outright.
Absolute and Gauge Pressure
P = P0 + ρgz | Pgauge = P − Patm
- P = Absolute (hydrostatic) pressure on a submerged object
- P0 = Ambient/surface pressure — not always atmospheric
- z = Depth below the surface
- Pgauge = Pressure measured relative to atmospheric pressure
- When P0 = Patm (open systems like a pool or tank), Pgauge simplifies to ρgz.
- 1 atm ≈ 101.3 kPa ≈ 760 mmHg ≈ 760 torr ≈ 14.7 psi ≈ 1.013 bar.
Pressure Types at a Glance
| Type | What It Measures |
|---|---|
| Atmospheric | Baseline pressure from Earth's atmosphere (decreases with altitude) |
| Absolute (hydrostatic) | Total pressure on a submerged object: surface pressure + fluid's own weight |
| Gauge | Pressure relative to atmospheric — a tire reading 0 is at atmospheric, not empty |
Common MCAT Trap
- A gauge pressure reading of zero doesn't mean zero absolute pressure — it means the system is at atmospheric pressure. A tire at '0 psi gauge' still holds ~14.7 psi absolute.
- Pressure is a scalar, not a vector — don't try to assign it a direction the way you would force.
- P0 in the absolute-pressure formula isn't automatically 1 atm — only assume that for ordinary open-air systems.
Quick Recall
An object has SG = 0.92. Does it float or sink in water, and why?
Why is pressure a scalar even though it's derived from force (a vector)?