Mass and Density

Definition of mass

Mass is a measure of the amount of matter in an object

The mass of an object is constant and does not vary from place to place

The SI unit for mass is kilogram (kg)

Tonnes(t)

Kilogram(kg)

Gram(g)

Milligram(mg)

1 t = 1000kg

1kg = 1000g

1kg = 1 divided by 1000 tonne

1g = 1000mg

1g = 1 divided by 1000kg

1mg = 1 divided by 1000g

An instrument used to measure mass is called a balance

Definition of weight

Weight depends on the mass of the object and the force of gravity. It can change as the force of gravity changes

For example, if an object is 6kg on earth, it is 1kg on moon.

The SI unit for weight is newton(N)

The instrument which is used to measure weight is called a spring balance

Definition of density

Density of a substance is defined as its mass per unit volume

Different materials have different densities

The SI unit for density is kilogram per cubic metre(kg/m to the power of 3)

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When the density of an object is less than that of the fluid(liquid or gas), the object will float.

When the density of the object is more than that of the fluid, the object will sink.

Some uses of density below

•The hydrometer. Measures the density of liquids. Used to test electrical strength of accumulator

•The lactometer. Used to test the purity of milk

•The plimsolls lines. Markings on the hulls of ships showing how deep the ships may be submerged in different types of sea water safely.

•Hot air balloon. Rises in the air. Hot air is less dense than air at ordinary temperature.

• Submarine. By pumping water in and out of it, the density of the submarine changes. Hence the submarine may float or sink in water.