When to say weight and when to say mass

By on Jul 21, 2009 in Physics |

I’ll try not to make this too much of  a rant. Having had a physics education over the years, I tend to pick up on people’s confusion of certain concepts. The most common one is that people don’t seem to be aware of the difference between weight and mass.

Roughly speaking, mass, is a measure of how much stuff there is of something, and we measure it in “Kilograms“. Whereas, weight, is how much downwards force an object exerts due to its mass, and we measure force in “Newtons“. So you see, it is an invalid statement  to say that something weighs 10 kilograms. Also, when you weigh something, you’re determining its mass, by measuring its weight. Scales are calibrated to read off in units of mass, by now much weight/force is being exerted on them.

Numerically, the difference between the weight and mass of a given object comes from the Earth’s gravity. Denoted “g“, it has a value of around 9.81 m/s2, that’s “meters per seconds squared” to you, which is the unit by which we measure acceleration. The formula to convert mass to weight is simply (yes there’s some maths!),

Weight = Mass * Earth’s Gravity
W = M * g

So next time you’re talking about how much of something you’re getting, you’re talking about mass, unless you’re talking about lifting or carrying something, then it’s weight. When you read your bathroom scales, that number isn’t your weight, it’s your mass. If you don’t remember this lesson for yourself, then remember it for all us poor balding physicists who are slowly tearing our hair out at such misuse of language!