6. Fluids & Solids

Question 1

Density

Answer 1

Density (ρ) – ratio of object mass (m) to its volume (v)

ρ = m/V

Unit: g/cm³

Question 2

Pressure &

SI unit

Answer 2

Pressure = Force per unit Area

P = F/A

SI Unit: Pascal = Newton/m²

Other units: 1 atm = 1.01 · 10⁵ Pa = 1.01 bar =

760 mmHg = 760 torr = 14.7 psi or lb/in²

Question 3

Relative Density &

Specific Gravity

Answer 3

Relative density RD - ratio of density of substance to density of given reference material.

• RD for gases – measured with respect to air at room temperature.
• RD for liquids – nearly always measured with respect to water at its densest called Specific gravity SG – how object behaves when it’s floating in water!

Question 4

Water density

Answer 4

Question 5

Archimedes Principle

Answer 5

ARCHIMEDES’ PRINCIPLE - Object which is completely or partially submerged in fluid experiences upward force F equal to weight (mg) of fluid displaced.

This is BUOYANT (FLOATING) FORCE: F_b = mg = Vρg

because volume V & density ρ is like mass of object but relates to fluids

Question 6

Specific Gravity

Answer 6

SG= ρ_substance/ρ_water = V_submerged/V_substance

Example: if SG = 0.90 = 90/100 then 90% of height of object would be submerged in water = less dense objects float in water.

Floating object is system at equilibrium: Force of weight (gravitational force) = buoyant force