This is not intended to be yet another popular book that offers... the latest news in physics. Still, it would be a pity not to show the links betwee… - Steven Weinberg

So much has happened in cosmology since the 1960s that this book... bears little resemblance to... Gravitation and Cosmology. On occasion I refer back to (..."G&C") for material that does not seem worth repeating... Classical general relativity has not changed much since 1972 (apart from a great strengthening of its experimental verification) so it did not seem necessary to cover gravitation... I provide a brief introduction in Appendix B. Other appendices deal with technical material... I have also supplied a glossary of symbols...

When the velocity and the acceleration are changing, we can define the instantaneous velocity or acceleration at any moment as the average values... over a vanishingly small time interval centered on that moment. Newton's Law actually relates the force to the instantaneous acceleration.

As a special case of Newton's Second Law, a body... when acted on by zero force, will experience zero acceleration—that is, it will move with constant velocity. Newton listed this... as the First Law... The Third Law... action equals reaction: If one body exerts a force on another... the second... exerts an equal force in the opposite direction on the first.