Gearheads

The Primaria kids continue to be fascinated by contraptions,
factories, and the like.  Thursday, the day before my visit, they
built contraptions using empty cardboard boxes, egg cartons, steel
cans, etc, plus a lot of imagination.  So I explored pulleys and gears
with them on Friday.

In my previous visit we built an elevator; that was more about the
principle of balance than about pulleys, but it did give them a basic
intro to pulleys. This time, I rigged up different pulley arrangements
to lift identical concrete blocks, using the monkey bars to hang the
pulleys.  One arrangement was just a single pulley at the top as you
might expect.  The second arrangement had the end of the rope tied at
the top, running down to an "upside down" pulley attached to the
block, and then back up to a pulley at the top which acted much like
the single pulley, just reversing the direction of the rope so that
the kids could stand on the ground and pull down on the rope to make
the block go up.  The kids tried both setups and compared the
difficulty of lifting the block.

The second arrangement is much easier.  I didn't expect the kids to
figure out why, but I did expect them to see that it had two pulleys
instead of one, or that it had a moving pulley rather than just a
fixed one.  Two of the four groups did not see this and required some
coaxing.  But I made a kind of game out of it, telling them that in
science we have to be very observant, asking them to watch carefully
as I pull each one slowly, etc.  I was happy to be able to frame it
such that they could gradually work toward the answer rather than just
have me give them the answer.

So why does the moving-pulley system make it easier?  I took lots of
very entertaining guesses on this one before having them observe the
motion again.  The moving pulley makes it so that if I pull my end of
the rope one foot, the weight moves up half a foot.  This means that
you only need half the muscle that you need with the fixed pulley.
(This is called "mechanical advantage" but I did not use that term.)
Then I asked how they could imagine making it even easier to pull.
Some of the groups digressed at first ("add a motor", "get a lighter
block") but we generally concluded that even more pulleys would be
better.  Kids love big numbers, and instead of suggesting four pulleys
some went straight to "a thousand pulleys!"  I had tried setting up
four pulleys, but the ropes got too twisted.  If you want to go to
four pulleys, I recommend buying sets of two pulleys already bolted
together side-by-side to avoid this twisting (parallel pulleys).  I
can't imagine how twisted the ropes would get with a thousand pulleys!

Next, we did gears.  They had already played a lot with
Gears!Gears!Gears! sets, but those are limited in terms of gear
concepts.  I ordered some bags of gears of very different sizes and
had hoped to mount them in some way which allowed for exploration, but
I ran out of time drilling holes at 8:45 Friday morning.  So this was
more of a demonstration than a hands-on activity, but that was ok
because it allowed me to use something I had only one copy of: the
book Get in Gear by Sholly Fisch, which is a really nice book.  Each
page describes a new gear concept and gives you the framework for
assembling it and seeing it work for yourself.

Before going to the book, I wanted to make sure they understood gear
ratios (although I didn't use that term).  I showed a little gear
turning a big gear in one of my homemade setups, and we counted how
many times we had to turn the little gear all the way around before
the big gear went around once.  In this case, it was about 3, because
the big gear had about 3 times as many teeth.  Conversely, turning the
big gear once makes the little gear go around about 3 times.  So if
you need to build a high-speed machine, hook a motor up to a big gear
which turns a little gear, and the little gear will go crazy fast.
And if you need to build a low-speed machine, hook your motor up to
the little gear, and the big gear will trun slowly.  We talked about
why people might need to build a low-speed machine.  This connects
back to the pulleys: when moving a heavy weight, low-speed is
better. (I left it at that without talking about forces; I think the
low-speed motion of the concrete block in the easy-to-pull setup was
the most effective and appropriate "proof" for this age group.)

On to the book.  I had noticed that the kids are paying attention to
clocks and starting to learn about time, so I started with the clock.
This was a natural segue from the gear ratio demo.  We want to make
the hour hand go slowly, so how do we do that with gears?  Attach the
hand to a big gear which is driven by a small gear!  And we want to
make the minute hand go fast, so how do we do that with gears?  Attach
it to a small gear which is driven by a big gear!  I was pleased that
the kids were able to guess these answers most of the time.  So here's
the clock in action:



Next, I showed them that gears are not limited to circular motion. Here is a rack gear in action:

Rack gears are used for turning circular motion into linear motion.  In addition to all kinds of machines, rack gears are used in steep mountain railways, where the track contains the rack gear and the engine carries and pushes on the circular gear.  (It's also used for rack-and-pinion steering; the pinion is the circular gear which meshes with the rack.)  We talked about what kinds of machines might need to do this kind of motion. Maybe squeezing grapes for grape juice, or printing presses.

And we can also set up gears to do a sweeping motion, by attaching something off-center:

The last thing we had time for was planetary gears, so called because little gears go around a bigger gear like the planets around the sun:

This is cool and could just be a work of art, but there are applications. Note that around the outside is what is basically a really big inside-out gear (difficult to see in the video because it's made of clear plastic). I went back to the homemade big+small gear setup and asked why it would be useful to put the small gear inside the big gear. Answer: to save space, if you need to make a small machine, like a pencil sharpener, a kitchen mixer, maybe an electric toothbrush.

Finally, we didn't get time to build the piece de resistance, but here is a machine which combs your hair and brushes your teeth at the same time:

The whole activity worked well.  I learned something about organizing kids, too.  Because there weren't enough pulley setups, kids had to wait, but there wasn't really a line because it was just a few kids waiting.  This led to a lot of confusion until Teacher Jessica brought "waiting chairs."  When the kids have to sit in chairs to wait, it is 100% clear who is next! 


If you want to see more, I recommend the video Gear Basics.