Question

How do I measure the mass of a brick, not the weight.

Answer 1

Push on it with a measured force F, then measure the resulting acceleration a. Then use Newton's Second Law:

m = F/a

Answer 2

Of course not. Force is directly measureable, and so is acceleration. Newton's Second Law is a definition of inertial mass.

Answer 3

By the way, force is not defined by Newton's second law.

Answer 4

What is it defined by?

Answer 5

Common sense? Force is something that causes an acceleration in a body that can move. It's directly experienced as a pressure or impulse. Why do we need a definition for a direct observation? That's like asking for a definition of length. Length is something we directly perceive, it needs no other definition than simply pointing to it.

Where we need definitions is of things that we cannot perceive directly, like inertial mass, or energy. These thins must be defined in terms of things we can observe, like forces and accelerations.

What you may be asking is what relationship force bears, mathematically, to the whole zoology of physics concepts. For example, force is a derivative of energy with respect to placement. But that's not a "definition" of force (if anything, it's a definition of energy), it's just the relationship between force and energy. That's because force comes first -- force is directly observable, energy is not.

This is not to say that you can't build up a coherent theory of physics starting from abstract principles and *deriving* the nature of force. That's certainly possible (at least, we hope so). But that still isn't really a definition of force, so much as stating where force fits in your abstract construction. Because science is fundamentally empirical: the proof of an abstract construction is its consistency with what's observable. That is, you may "derive" force if you like from abstract ideas (like energy), but your ideas must end up proving that force has the properties we know it has from direct observation. So the "derivation" can't possible come up with any new properties -- it's more like a proof than a derivation, perhaps, speaking mathematically.

Answer 6

I'd agree with that. If there's something to be defined by F = ma, it's the inertial mass.

Answer 7

It's a fair cop.

Answer 8

Measure its mass on a scale. Scales are set to accept that the acceleration due to gravity is 9.81 m/s. So mass is the amount of 'stuff' there is, not its force. But knowing the force gives the mass.

Answer 9

Did you learn F = ma ?
If so, you can re-arrange the equation to read:
m = F/a where m is an unknown mass

Answer 10

OK let's put my brick in outer space, now how would measure its mass?

Answer 11

Do what Carl said in the first response to this question.