First published in 1962, many of the problems in this book started as examination questions in Part I of the Natural Sciences Tripos, which is taken at the end of the second year at Cambridge. They have suffered some changes since then, and have been supplemented by specially invented problems, but the general level is the same. The university teacher, however, should not imagine that our purpose in publishing this collection is to provide him with a ready store of examination questions. We are much more concerned to help the serious student to understand physics, and it is his needs that we have tried to bear in mind throughout.