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Everyone (well… almost everyone!) knows something about weight.  We talk about things being heavy, and other things being light. Most of us have an idea of how much we weigh – which is an indication of how heavy we are. When we stand on a weighing scales, either the needle or the numbers indicate our weight.

Usually we talk about our weight in Kilograms (kilos). A ten-year old could weigh 40 kilos, but as we grow older we grow heavier.   Some adults may weigh 100 kilos or more.  

We use kilos and the smaller weight value known as grams in lots of everyday situations, for example when buying fruit or vegetables.   Everyone knows what we mean. Scientists, however, see a difference between ‘mass’ and ‘weight’. Most of the time when you are ‘weighing’ something you are actually measuring its mass.  

Weight or Mass?

Mass is actually the amount of matter or ‘stuff’ in a thing, and it is what is properly measured using kilos and grams. Weight, on the other hand, is slightly different.  It is a force, not a mass.  Actually it is defined as ‘the force of gravity on an object’. This means weight varies, depending on where you are (on Earth, on the Moon, in space) – because gravity varies like that. Mass, on the other hand, is the same wherever you are.

When scientists measure weight, therefore, they are measuring a force, and they use a measure called the NEWTON (called after a famous scientist of the 1600s – see the section ‘Famous Scientists’).  A mass of one kilogram on Earth is approximately equal to a force (or weight) of 10 Newtons .

This is a bit complicated and you will learn more about the difference between mass and weight in second level school. So for the parts of this website which focus on how gravity affects weight, we will   mainly just use numbers when weighing things (without always saying whether they are grams, kilograms or Newtons ).