Axiom 1. Definition: a machine is a physical object through which energy can flow, and because of its shape, the arrangement of its parts, and other physical characteristics, causes the energy to be changed as it flows through.

Matter is material: it has weight, it takes up space. Energy is non-material: It does not have weight and it does not take up space.

Machines are all about the interaction between Matter and Energy.

A machine is a designed piece of matter that interacts with energy in a specific way and for a specific purpose.

For example, a mirror is a machine. When light hits a mirror it bounces back at an equal but opposite angle. Energy flows to the mirror, interacts with the matter that makes up the mirror, and flows out of the mirror. The mirror has been designed by careful choice of the material it is made of (eg, silver), the way it is arranged (eg, very flat), and other physical characteristics (eg, it is highly polished).

Machines are not energy, but energy flows through them: they manipulate energy. If you throw a rock, you impart energy from your body to the rock. It flies through the air and hits something, say a car door, which is then dented. The rock and the door are not energy. In fact, energy (or "force") is invisible: you can't see it. It is not matter. You can only see the effect it creates on matter when it interacts with matter. It makes the rock fly, it makes the door dent. These are results of energy interacting with matter.

Suppose we tie a rope to a heavy object and then throw the rope over a high tree branch. Now when we pull down on the other end of the rope, the object rises up, to a point much higher than we could have lifted it without this machine. The rope and the tree are a machine: you apply energy at one end and it comes out at the other. And this occurs for no reason other than the physical arrangement of the parts. A rope lying on the ground next to a tree would not work.

Thus it is the nature of the object, its shape, the arrangement of its parts, the kind of matter it is made of, that is what causes the energy which flows through it to change. Energy cannot be created or destroyed: but it can be changed in form and location, by interacting it with matter. And controlled interactions designed by humans to create useful effects are called "machines".