Simple Nature - Light and Matter

Self-check D.since the derivative of the constant u is zero. Thus an acceleration,unlike a velocity, can have a definite physical significance to allobservers in all frames of reference. If this wasn’t true, then therewould be no particular reason to define a quantity called accelerationin the first place.self-check DThe figure shows a bottle of beer sitting on a table in the dining car of atrain. Does the tilting of the surface tell us about the train’s velocity, orits acceleration? What would a person in the train say about the bottle’svelocity? What about a person standing in a field outside and looking inthrough the window? What about the acceleration? ⊲ Answer, p. 9211.4 A Preview of Some Modern Physics“Mommy, why do you and Daddy have to go to work?” “To makemoney, sweetie-pie.” “Why do we need money?” “To buy food.”“Why does food cost money?” When small children ask a chainof “why” questions like this, it usually isn’t too long before theirparents end up saying something like, “Because that’s just the wayit is,” or, more honestly, “I don’t know the answer.”The same happens in physics. We may gradually learn to explainthings more and more deeply, but there’s always the possibilitythat a certain observed fact, such as conservation of mass, will neverbe understood on any deeper level. Science, after all, uses limitedmethods to achieve limited goals, so the ultimate reason for all existencewill always be the province of religion. There is, however, anappealing explanation for conservation of mass, which is atomism,the theory that matter is made of tiny, unchanging particles. Theatomic hypothesis dates back to ancient Greece, but the first solidevidence to support it didn’t come until around the eighteenth century,and individual atoms were never detected until about 1900.The atomic theory implies not only conservation of mass, but acouple of other things as well.First, it implies that the total mass of one particular elementis conserved. For instance, lead and gold are both elements, and ifwe assume that lead atoms can’t be turned into gold atoms, thenthe total mass of lead and the total mass of gold are separatelyconserved. It’s as though there was not just a law against pickpocketing,but also a law against surreptitiously moving money fromone of the victim’s pockets to the other. It turns out, however, thatalthough chemical reactions never change one type of atom into another,transmutation can happen in nuclear reactions, such as theones that created most of the elements in your body out of the primordialhydrogen and helium that condensed out of the aftermathof the Big Bang.Second, atomism implies that mass is quantized, meaning that68 Chapter 1 Conservation of Mass