Physica requires Minecraft Forge to load. You can find this here: https://files.minecraftforge.net/
Physica is separated into a core mod that includes the basic components for mod, and several other mods that focus on different topics. This core mod is required for all of the other mods to work, but none of the sub-mods are dependent upon each other. An important thing to note is that the Physica core mod is released alongside the other sub-mods. What i mean by this is that version 1.2.3 of Physica Core is compatible with version 1.2.3 of Physica Nuclear Physics. Other than this just drop the mods you want to use into your “mods” folder in the minecraft directory you use and you’re good to go!
You can download physica here: click this link