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Different representations for different situations.
This reminds me another 'representational' answer to another 'Silly
Question' that occasionally comes up in an intro Physics environment.
'Why does a mirror reverse left and right but not up and down.'
I've heard 'serious' answers that 'explain' it in terms of our having
two eyes - side-by-side - and our 'brain' figures it out and other
such foolishness.
An easy demonstration of what's going on is (as JD sez) use the
representation that works with a given situation.
If you look into a horizontal mirror (either on the floor or on the
ceiling) it is clear that only up and down are switched. North,
South, East and West are unaffected.
Same goes for a vertical mirror. If it's in the N-S plane then it's
only the East-West directions that are switched. Up and Down stay the
same as do North and South.
Use the representation that works in a given situation.
.
At 4:07 PM -0700 2/14/12, John Denker wrote:
It is difficult or impossible to_______________________________________________
express things in terms of northward, southward, eastward, westward,
or other heading-related notions ... but we don't need to do that.
There are other (better) representations we can use.
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