**Kilogram** (kg)

- A
**kilogram**is defined as the equivalent mass of water (at its most dense state i.e. 4°C) occupying a volume of one thousandth of a cubic metre.(i.e. 1 L)

- Mass and weight are often confused.

Mass is the amount of matter of an object and is constant, while weight is the gravitational force acting on that object or mass and dependent on the gravitational strength.

That is, on the moon we weigh less then on earth but are still composed of the same amount of matter or mass.

The unit of force is a Newton (**N**- a SI derived unit).

The unit of mass is a Kilogram (**Kg**- a SI unit).

A Newton (**N**) is the force required to accelerate one (1)**kilogram**(kg) at one metre per second, every second (**kg m/s**)^{2}

Since the earth's average gravitional force per mass is constant , involving a average acceleration of 9.8m/sec^{2}, we use**weight**(gravitational force acting on our mass) to determine relative masses.

When you weigh yourself, the scales covert weight to read out in mass units (kg) and not gravitational force units (Newton/kg)

- Volume units are derived from the SI unit kilogram

One litre (L) is the volume occupied by one kg of water in its densest state (@ 4° C).

It is therefore equivalent to 1000 ml, (i.e. 1000 cm_{3}or 1 dm_{3})