Physics Flashcards — Density & Speed
Density
What is it?
Definition: Mass per unit volume of a substance.
ρ = m / V
ρ = m / V
Units: kg m⁻³ (SI) — Commonly g cm⁻³ for school problems.
Example: A block of mass 2 kg and volume 0.001 m³ → ρ = 2000 kg/m³.
Relative Density
Also called Specific Gravity
Definition: Ratio of density of a substance to density of water (at 4°C).
RD = ρ_substance / ρ_water
RD = ρ_substance / ρ_water
Dimensionless (no units).
If ρ_substance = 2500 kg/m³ and ρ_water = 1000 kg/m³ → RD = 2.5.
RD in Terms of Masses
Using equal volumes
Idea: For equal volumes, RD = mass of substance / mass of equal volume of water.
RD = m_substance / m_water (for same V)
RD = m_substance / m_water (for same V)
If 50 g of substance occupies same volume as 20 g of water → RD = 50 / 20 = 2.5.
Useful in lab: measure mass of object in air and mass of equal volume of water (or use overflow method).
Speed
Basic kinematics