Introduction
That an object sinks, floats or hovers has nothing in particular to do with its mass, but its density. The density of an object is equal to its mass divided by the volume. Solid substances with a density smaller than the fluid will float. Solid substances with a density greater than the fluid will sink. When the density is equal, the solid substance will hover. A hol object floats more easily than a massive object because its total density is smaller. The bigger part of a hol object contains air, which has a relatively small density.
note: If we take for example: a massive iron cube with an exact volume of 1 dm³. In air this cube has a weight of 7.9 kg. Immersed in water the cube will only weigh 6.9 kg. The weight of the cube has been diminished by the weight of the water that has been moved by the cube. This is the weight of 1 dm³ water, which is 1 kg.
Definitions
Mass
The property which is used to measure how easy it is to accelerate an object, is called mass.
The mass has nothing to do with the amount of matter an object contains.
Symbol: m Entity: kg
Force
1 Newton is the force that needs to be given to a mass to give it a speed of 1 m/s or to give that mass an acceleration of 1 m/s²
Symbol:F Entity:Newton (N)
Weight
The weight of a body is the strength with which the ground attracts the body.
Since the weight is a strength (or force), it is measured in Newtons.
It has been measured that 1 kg has a weight of 9.8 N
Symbol:G Entity:N
Difference kg and N
People regularly mix up Kg and N. N is a force and Kg is the mass. My mass is 72Kg but my weight is 720 N (= a force).
Formulae
Floating-power: FD = V . ro . g with
V = the volume of the moved liquid
ro = the density of the liquid
g = 9,81 m/s2 (the acceleration when we fall)
The gravity on an object: FG = V . ro' . g with
V = the volume of the moved liquid
ro = the density of the object
g = 9,81 m/s2 (the acceleration when we fall)
Now, there are 3 possibilities:
1/ FG > FD -> ro' > ro -> The object will sink.
2/ FG < FD -> ro' < ro -> The object will float.
3/ FG = FD -> ro' = ro -> The object will hover.
The density of some objects
gas -> 0,6 kg/m³
air -> 1 kg/m³
cork -> 130 kg/m³
soft wood -> 350 - 550 kg/m³
strong wood -> 700 - 900 kg/m³
oil -> 900 kg/m³
clean water -> 1000 kg/m³
salt water -> 1025 kg/m³
milk -> 1040 kg/m³
glas -> 2400 - 2900 kg/m³
aluminium -> 2700 kg/m³
rock -> 2500 - 3000 kg/m³
iron -> 7900 kg/m³