Physics 101- Introduction to Mechanics
February 27- Introduction to Energy

Conservation Laws

The following analogy is from Richard Feynman's book Six Easy Pieces.  I connat recomend this book (and Six Not-So-Easy Pieces) enough.  For anybody intersted in science they are classics.

Consider the case of a 10 year old who has 28 indestructible playing blocks, the weight of each block being 0.1 kg.  Everyday, the boy's mother counts the blocks, and it always adds up to 28.  One day she finds only 27 blocks, and by looking around she finds one lying under the bed.  A few days later she finds only 26 blocks.  On searching the room she finds a toy box; when she tries to open the toy box, the boy screams "Don't open my toy box".  Being a smart mother, she weighs the box when she has 28 blocks and finds that the empty toy box weighs 0.5kg.  Henceforth, when she cannot find all the 28 blocks, she weighs the toy box to determine how many blocks it has.  The mother now a method of counting blocks no matter where they are.

The blocks can be located in a number of different places ­ the child's room, his box, the bathtub, the backyard, etc.  But, the number of blocks never changes.  The mother simply must determine where they are, but the number of blocks is conserved.

Similarly, all that a physical process can do is to transform energy from one form into another.  Hence, for energy we have the following expression:

Total energy of the system = (energy in form A) + (energy in form B) + (energy in form C) + etc.

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Spring 2002