Major West African empires formed around what?

The history of West Africa has been commonly divided into its prehistory, the Iron Age in Africa, the major polities flourishing, the colonial period, and finally the post-independence era, in which the current nations were formed. West Africa is west of an imagined north-south axis lying close to 10° east ... The Ghana Empire may have been established as early as the 7th century ...

Explanation:

motion can be transfered from one object to another by having an objetct in motion push or hit it