join two broke pieces of the same material

scientifico

Hello, if I broke for example a piece of wood in two pieces I give the atoms enough force to separate in the break point with the others, so why putting the two pieces together and pressing them the atoms don't join again ?

Thank you!

mfb

This is possible with metal, but it has to be extremely clean and in a vacuum.
If you break wood, you modify the surfaces, if you try to combine them again you get a lot of ripples which do not fit. And on a molecular level, you need energy to combine the parts as well, which would be released as heat afterwards.

scientifico

why would I need energy if atoms attract each others?

turbo

Deformation of the material during breakage would prevent you from joining them cleanly afterward.

mfb

Atoms do not generally attract each other. Some do, some do not. In many cases, you have to add some energy to the system first, even if you can extract more energy afterwards.

Think of a wood fire: It burns, and it can release a lot of heat. But if you just place wood somewhere, it does not begin to burn. You have to add energy first.