Balance, and floating vs sinking

Today in the 1-2 grade room we had a blast with some of the ideas we
need to use in making the water feature.

First, balance.  I brought in two-meter-long sticks on pivots, along
with sets of weights of various sizes, and had the kids hang weights
in different places and then see where they had to place other weights
to balance it out.  They quickly discovered that a small weight can
balance a large one, IF it is placed at the end of a long arm.  This
was a really good exercise because, in contrast to some of our
previous ones, I had enough equipment for each child to explore
completely on his/her own. 

The pre-snack period culminated with two capstone events:
(1) I gave the kids a worksheet in which I drew balance beams
with a weight on one side (varying the size and position of the
weight), and they had to draw the weight (size and position) they
would put on the other side.  Mostly they got it right, and in the few
cases where there was confusion we had the equipment right there to
check if their drawing represented reality.  (2) I demonstrated how
balance facilitates rotation.  You can see a video I made about this
demo at the end of this blog post from last year.  As kids went to break,
some of them commented how this demo is like the Moon going around the
Earth, and asking whether the Earth wobbles a little as it does so.
The answer is yes, and so does the Sun as the planets (Jupiter has the
biggest effect) go around it.  Therefore, if you saw a star which was
wobbling, what could you conclude about it?  Right, it has planets!
This is really how astronomers do it; the vast majority of planets are
too faint to see directly given the glare of their host stars.

Post-snack, we switched to fluid mechanics.  We started by reviewing
what we learned about pressure last time, focusing on why water
doesn't fall from a straw when you cover the top with your finger.  I
then showed the same idea in slightly different form: with two 2-liter
soda bottles screwed together, water does NOT fall from the top one to
the bottom one (it may drip, but it doesn't make the waterfall you
might expect in an open-bottle situation).  The water doesn't fall
because for the water to go down, the air in the bottom bottle has to
move up, and the two get in each other's way.  We then figured out how
to make them not get in each other's way: swirl it to make a "tornado
in a bottle."  The air goes up through the middle while the water
swirls down around the outside.

We then took some time for each kid to make his/her own tornado in a
bottle, with the option of coloring and/or glittering the water.  This
was great fun; the kids were really into it and came up with some
pretty (and/or Halloweeny) combinations. 

Next, we studied floating and sinking, following more or less the
script from one of my Primaria sessions last year (adding a bit of
sophistication such as introducing the word density).  But we had time
only to get to the egg in the salt water.  We'll do the rest next time.

At the last minute, we stumbled into a nice connection between the egg
and geology.  Teacher Pa said that the way to tell if an egg has gone
bad is to see if it floats (in non-salted water).  Linus had said just
5-10 minutes before that pumice is a rock that floats because it has
lots of gas bubbles in it.  So the connection is that an egg which
floats (without the help of salt) probably has gas bubbles in it,
which clearly is a sign that it's going bad.