Mass explanation: I’ve always been told that mass was not the same as weight, and that grams are the metric unit of mass. But grams are a measurement of weight, so am I stupid, was it was explained to me wrong, or is science just not make sense?
In: 140
There are two distinct notions of mass.
“Inertial mass” is a measure of how hard it is to change the current speed of the object. (A lorry has more inertial mass than a car; it’s harder to slow it down when it’s moving).
The other notion is “gravitational mass” – how hard it is to move an object against gravity. (A lorry has more gravitational mass than a car – it’s harder to lift it up).
The actual effort required to “lift” an object with gravitational mass through a particular gravitational field is called “weight”. (A lorry has more weight on earth than on the moon. It has the same gravitational mass, but the gravity is weaker on the moon so it’s easier to lift).
Essentially, gravitational mass is an *intrinsic property of just the object*. Weight is this intrinsic property of the object *combined with the strength of the gravity it’s sitting in*.
Interestingly, the numerical value of the inertial and gravitational masses is exactly the same, and we don’t really know why. In Einstein’s general relativity, their equivalence is taken as a postulate called “the principle of equivalence”. People have proposed some theories (e.g. string theory) that allow you to derive the equivalence from more fundamental axioms, but we don’t know the real answer for sure yet.
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