Mass

Take three identical paper cups and fill each one half full: cup A with water, cup B with sand, cup C with pebbles. Predict which will be heaviest. Then weigh each on a balance and compare.

How heavy or light something is, is measured as a property called mass. The heavier object has more mass; the lighter one has less. In everyday talk people often say 'weight' when they mean mass, because we find mass by weighing. You will learn how mass and weight differ in higher classes.

Mass is measured in grams (g) and kilograms (kg). The kilogram is the SI unit of mass. An eraser has a mass of a few grams; a school bag, a few kilograms.

Writing mass correctly:

• kg is written in small letters, with no space between k and g.
• No full stop after it (except at the end of a sentence).
• A space between the number and the unit.

So 7 kilograms is written 7 kg, never 7 kgs.