VIS U AL P HYS ICS
S ch ool of P h ysi cs
U n i v er si t y of S yd n ey Au st r a l i a
DENSITY ρ
Visualisation
How does density relate to hot air ballooning?
How big can you make an air ship?
Definition (ρ Greek letter rho)
!
m
ρ=
V
m
V
mass of object
volume of object
Units
SI unit
kg.m-3
Other units g.mL-1
1 mL = 1 cm3 = (10-2)3 m3 = 10-6 m3
1 g = 10-3 kg
1 g.mL-1 = 103 kg.m-3
?
• If you had equal volumes of air and water, why is the
water so much “heavier”?
• Why does a cork float? (average density of an object <
density of fluid ⇒ object floats)
• Why does hot air rise? (convection currents)
• How do atmospheric high and low pressure systems
develop?
• Why is it easier to float in salt water?
• Why does ice float?
Density values (kg.m-3)
Interstellar space
10-20
Best laboratory vacuum
10-17
Air (20 °C and 1 atm)
1.21
Styrofoam
100
Ice
917
Water (20 °C and 1 atm)
998
Salt Water (20 °C and 1 atm)
1024
Whole blood
1060
Earth (average)
~5500
Iron
7900
Mercury
13 600
Uranium nucleus
1017
Neutron star (core)
1018
Mathematics
gold
m1 V1
ρ V
m=ρV
ρgold = m1 / V1 = m2 / V2
gold
m2 V2
ρ m
V=m/ρ