# Homework Help: Mass' versus Amount of matter

1. Jan 21, 2012

"Mass' versus "Amount of matter"

1. The problem statement, all variables and given/known data

I just started a thermo-fluid science course and am confused with a table showing common dimensions

2. Relevant equations

Dimension................Unit
Mass......................kilogram (kg)
Amount of matter.....mole (mol)

3. The attempt at a solution

I thought mass was the amount of matter something had. Then what's the difference between a kilogram and a mole?

2. Jan 21, 2012

### Staff: Mentor

Re: "Mass' versus "Amount of matter"

A mole is a count of particles (atoms, molecules, etc.) comprising something. Mass refers to that something's inertia and/or response to a gravitational field.

3. Jan 21, 2012

Re: "Mass' versus "Amount of matter"

I'm thinking back to my 4th grade science teacher's saying: "Matter is anything that has mass and takes up space."

The "has mass" part means that the matter resists being pushed by an external force (interia) and that the matter can be tugged by gravity.

Is this correct?

The "takes up space" part means that there's no such thing as a "point mass" in which something could have matter but would be physically dimensionless, right? It's funny how often we use point-mass approximations in basic physics courses.

4. Jan 21, 2012

### Staff: Mentor

Re: "Mass' versus "Amount of matter"

In a simplistic way, yes, it is correct. Quite suitable for 4th grade science. At deeper levels physics recognizes three types of mass: Inertial mass (the resistance to be accelerated), corresponding to the m that appears in the formula f = ma; and active and passive gravitational masses that appear as M and m in the formula f = GMm/r2, where f is the force that M produces on m. In practice, thanks to the Equivalence Principle, all three masses have the same numerical value for all three cases.

This is another one of those things that gets modified by a deeper look. It turns out that certain fundamental particles, like the electron, are point particles to the very best of our ability to measure. For these we treat them as point particles and place an upper bound on their possible size (experiment shows that they cannot be larger than this, usually fantastically tiny, size).

5. Jan 23, 2012