Density is defined as mass per unit volume and relative density compares the density of a substance to that of water. According to Archimedes' principle, an object will sink if the weight of water displaced is less than the object's weight, and float if the displaced water's weight equals the object's weight. For an object floating freely in water, the upthrust equals the weight of the object. Floatation only occurs when the upthrust equals the weight of the object.

1. PHYSICS
LIQUIDS AT REST
Short revision series

2. Density
 Mass per unit volume
 ρ= mass/vol = m/v
 Relative density = density of a substance in
relation to density of water
= density of the substance
density of water
= mass of substance
mass of equal vol of water
= weight of substance
weight of equal vol of water

3. Archimedes’ principle
 If the weight of the water displaced is less
than the weight of the object, the object will
sink.
 Otherwise the object will float with the
weight of the water displaced equal to the
weight of the object

4. Archimedes’ cont.
 T = W – U
 U = W – T
 U= W if the bucket floats freely.
U
W
T

5. Floatation
 FLOATATION can only occur When U = W
 Applications of this principle: Ships, hot air
balloons, iceberg