# Mass and Magnets

Q:
Mass is defined as the measure of resistance to change. when a magnet is attracted to a substance does its mass change?
- Dominique (age 10)
Rocky Mount NC USA
A:
Let me try to get more specific about that definition first, then go on to your question.

We like to define mass by comparing the acceleration of an object to the force applied to it. The acceleration measures how quickly the velocity is changing, the force measures how much something is pulling or pushing on the object. The mass is the force divided by the acceleration. So it is "the resistance to change", but it helps to say what type of resistance to what type of change.

For all practical purposes, the mass of a magnet doesn’t change at all when it’s attracted to another substance. You could test this by putting magnets in a box, and seeing if the box was easier to push when the magnets were together or apart. Since the same mass also determines the weight, you could make a much more accurate measurement by weighing the magnets apart and together.

Someday, if you go on to study physics, you will learn about some very tiny changes in mass when two magnets pull together, but they are way, way smaller than anything you can measure with an ordinary scale.

Mike W

(published on 10/22/2007)

## Follow-Up #1: Relativity for ten-year olds?

Q:
Why don't you just explain the change in the relativistic rest mass of the magnets when they're attached instead of just saying "someday when you study physics. . ."?
- Solletica
Saint Paul, MN
A:
We thought that would be way past what a ten-year old could follow.

Mike W.

(published on 08/04/2012)

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