## Physics Questions People Ask Fermilab

Definition of Matter

How do scientists define matter and how can you demonstrate that a piece of chalk has mass?

Ethan

Dear Ethan,

All the "stuff" around us is made of matter. Physicists define matter as something that has mass and comes in different forms: solid, liquid, gaseous.

But when does a particle or an object have mass? It has mass when it resists a change in its direction or speed when a force is applied. In other words: Massive objects prefer to maintain the motion they are in. On earth this is difficult to understand, as massive objects always tend to slow down because of friction. In space, however, out in the middle of nowhere in the universe, a massive object always keeps going at the same speed and in the same direction - unless a force is applied. Applying a force, you can slow down or speed up a massive object, or you can bend its path. You cannot slow down a massless object.

The laws of physics indicate that it is impossible to change the speed of massless things. The most familiar massless thing is light. And light always travels at the speed of light. No matter what force is applied, light keeps going at about 300,000 kilometers per second.

With regard to the piece of chalk: How do you prove it has mass? Hold it three feet off the ground and let it go. You know that earth exerts a gravitational force. If the piece of chalk has a mass, then the force will act upon it, speeding it up. So observe the piece of chalk. If it falls, it has mass.

Here is some background information on mass and matter:
http://whatis.techtarget.com/definition/0,,sid9_gci549408,00.html

Classification of matter: