AP Physics Study Packages AP Physics Fluids | Page 3

Density

Density – mass per unit volume. We’ve used d as it’s symbol in all your other science classes, physicists use the Greek letter “rho”, ρ, since we use d for distance all the time!

ρ = m/v = mass/volume

• It follows that mass = ρv and weight = mg = ρvg

• The SI unit for density is kg/m3

[often reported in g/cm3 and never forget that 1cm3= = 1 ml

∴ a density in g/cm3 should be multiplied by 1,000 to give the result in kg/m3

Temperature does have a slight effect on densities due to expansions and contractions.

Pressure

Pressure–Force per unit Area where the force is acting ⊥to the surface area

P = F/A

The SI unit is the Pascal [in honor of Blaise Pascal...more to come about him]

1 Pa = 1 N/m2 = 1.45 × 10−4 lb/in2 (psi)

A 60 kg person whose two “feetsies” cover an area of 500 cm2 will exert a P of

Same person stands on one foot and the area is cut in half which doubles the pressure,

∴ P = 24,000 N/m2

Specific Gravity – the ratio of ρ : ρ water @ 4° Celsius

ρ water = 1.00 g/cm3 = 1,000 kg/m3

That means that the specific gravity, SG, is the density without units IF the density is reported or first converted to g/cm.