The Physics (Greek: Φυσικὴ ἀκρόασις Phusike akroasis; Latin: Physica, or Physicae Auscultationes) is a treatise by Aristotle in which he sets out the fundamentals of his philosophy of nature. For Aristotle, a thing's nature (Greek physis) is the source of its motion, something he analyses in terms of his famous theory of the four causes (material, efficient, formal, and final).
Free online texts
Boston University: Physics, translated by R. P. Hardie and R. K. Gaye. HTML format.
Greektexts.com/Internet Archive: Physics. HTML format.
Internet Archive: Works of Aristotle Volume II. Translated into English under the editorship of W.D. Ross (1908). Multiple formats.
Internet Archive: Aristotelis Opera. Greek Bekker edition (1837). Multiple formats.
Internet Classics Archive: Physics, translated by R. P. Hardie and R. K. Gaye. HTML and TXT formats.
Remacle.org: Physique d'Aristote, translated into French by Barthélémy Saint-Hilaire (1861).
University of Adelaide (Internet Archive): Physics, translated by R.P. Hardie and R.K. Gaye. EPUB, HTML and MOBI formats.
Dominican House of Studies: Commentary on Aristotle's Physics, by Thomas Aquinas.
History of Philosophy without any gaps: Form and Function: Aristotle's Four Causes - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Let's Get Physical: Aristotle's Natural Philosophy - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Richard Sorabji on Time and Eternity in Aristotle - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: God Only Knows: Aristotle on Mind and God - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Aristotle - Motion and its Place in Nature, by Joe Sachs.
Librivox: Physics - public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Natural Philosophy, by Istvan Bodnar.
The Great Conversation: Further reading at Tom's Learning Notes.
Aristotle: On the Heavens.
Plato: The Timaeus - A key dialogue for Plato's natural philosophy.
Ancient Greek resources: Learn to read Greek classics in the original.