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Re: [Phys-l] representing position and orientation

Right. Students just get to see me holding the photocopy, but they try to
"hand wave" to get the answer. It's interesting to see how they think, and
some of them change their answers several times!

We also discuss the word AMBULANCE and the positioning of the letters. You
can write these with a marker on a piece of glass to show what's going on.

Do you have the link to that cartoon?


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
Part of the problem is that books always talk about the left-right
reversal
of mirrors, but the obvious front-back reversal is never mentioned. In
reality if you put L and R on your palms and look at it in a mirror, the
mirror image has them on the same side, so is there really a L, R
reversal?
Don't you see the same thing as if you were looking from the back side,
when
you look at writing? If we were Australian aborigines we would not appeal
to the R-L reversal, but rather to the compass point reversal as already
noted.

Incidentally Sidney Harris has a funny cartoon on his website showing
Alice
on the other side of the mirror trying to get out, and the legend is that
she can't get out because some replaced the glass with Lexan.

I assume the students are not allowed to handle the photocopy. All you
need
to do is put your hand over it to see which hand it was. Of course you
have
to be careful to put the hand in the same position so it looks the same as
the photocopy, with no reversals.

Of course you can make a non reversing mirror if you are careful to put
the
two plane mirrors at perfect right angles. As I recall there were some
cheap children's toys that did things like that. Non reversing mirrors
are
very perturbing because you are used to seeing the asymmetries in your
shaving mirror. When they are reversed you look lopsided.

John M. Clement
Houston, TX


Mirrors are not well understood by most people, even my best
students. You
really have to do labs and ray tracing to figure out
how/where images form
in plane mirrors.

I show a photocopy of my palm each year and ask the kids
which hand it is.
I never get a consensus. Same thing with adults!

I have them look at the vertex of two perpendicular mirrors
for a double
image reversal. This is how others see you. They really like this!
Educational Innovations sells such a mirror, and it is very expensive.


Forum for Physics Educators
<phys-l@carnot.physics.buffalo.edu> writes:
Those of us who are old enough can remember textbooks that
referred to
images in a plane mirror as being "erect but perverted".
Fortunately,
that choice of words has fallen out of favor - and Feynman's insight
simply makes it a mute point. We still have to struggle with
how to say
"Uranus" though.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Daniel L Macisaac
Sent: Wednesday, February 15, 2012 10:12 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] representing position and orientation

This very nice problem is discussed in a Feynman video at
http://www.youtube.com/watch?v=msN87y-iEx0
and the discussion may surprise. Dan M


On Feb 15, 2012, at 10:07 AM, chuck britton wrote:

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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