An object in motion...

So it's time for the elementary kids to learn Newton's laws of motion.
The key for good demos and hands-on activities is getting rid of
friction, so we can see how objects behave in the absence of forces.
I usually use hover pucks, devices that look like very big hockey
pucks but have a fan inside so they float on a cushion of air, like
air hockey without the table.  (They are also sold under the brand
name Kick Dis.)  But I decided we would have more fun and soap up a
table so that we could slide anything without friction.  The night
before, I started soaping tables to see how frictionless I could get
them.  The result was a lot of frustration.  I could never really get
them frictionless, not close enough to make convincing demos.  I even
tried soaping a large sheet of glass (one of the mirrored sliding
doors on my closet) and that was better, but still not good enough.
It seems that no table or mirror is quite flat enough; objects will
glide along a bit, but then hit a high point and stop.  So I ended up
going to bed late and frustrated, with some mess still to clean up in
the morning.  Such is the life of an educator.

So Friday morning quick I went to work and picked up a bunch of
hoverpucks.  (I also bought some donuts for the donutapult; see
below.)  I had already picked up some carts which looked like very
large skateboards.  On a smooth floor or sidewalk, these are
reasonably frictionless and safe to kneel on.

After scouting the school to find the smooth surfaces, I set up a
three-stage plan for each group.  We started at the long, smooth table
with the hoverpucks.  I asked the kids what they knew about friction,
and then I asked them to rub their hands together to feel friction.
Then I slid a switched-off hoverpuck on the table (it went a very
short distance) to show that friction is why it's hard to slide
things, and I asked them for ideas to get rid of friction.  After
entertaining various ideas, I asked them to make predictions for how
the hoverpuck would move after I switched it on and pushed it toward
the other end of the table.  Then I did just that, and it went in a
perfectly smooth straight line.  So this makes it clear that, in the
absence of forces, objects in motion continue their motion (in a
straight line at constant speed).  This is Newton's first law of
motion, but I didn't ask them to remember that.  We had too many cool
experiments to do, and I can count on the regular teachers to review
the terminology several times!

So I seamlessly continued with the hoverpuck.  I asked a child seated
along the middle of the table to give the puck a sideways tap as it
passed her down the long axis of the table.  First time with a small
tap, then with a larger one, and each time I asked the kids to predict
the subsequent motion.  This sequence shows a few things.  First, that
a bigger force (or tap, or push) causes a bigger acceleration (change
in motion, whether it be a change in speed or direction...mostly
direction in this case), which is part of Newton's second law.
Second, a force changes the motion only while the force is being
applied.  The tap changes the direction of the puck, but only while
the tap is applied.  After the tap, the puck follows its new direction
in a straight line.  A one-time tap cannot make it keep curving
around.  This reinforces the first law: while there are no forces, it
goes in a straight line at constant speed.

To wrap up the hoverpuck activity (which was only the first of the
three stages I had planned), I gave each child a hoverpuck and asked
them to figure out how to make it travel in a circle (which is
distinct from spinning).  They needed this bit of playing to relieve
the wiggles, because to this point it had been mostly demo, with some
assistance from 1-2 kids.  After several minutes, we discussed how the
only way to make the puck go in a circle is to keep your hand on it
and move your hand in a circle.  In other words, circular motion
requires a continuously changing direction of motion, and therefore a
continuous force.  This bit isn't strictly necessary if we just wanted
to do Newton's laws, but it connects to the next stage and some other
interesting ideas in the next paragraph.

Second stage: we went outside and I made a bagel-on-a-string go around
in circles over my head.  I asked them to imagine what would happen if
the string broke.  If they really got Newton's first law, they would
answer that it would fly off in a straight line, but of course most
people don't grasp it that well after just the first demo. So some
said it would fall straight down, a few said it would fly off in a
circular motion, etc.  So now comes the really fun part.  It's not
practical to cut the string, so instead I tie a donut to a string, and
the string cuts its own way through the soft donut as I whip it around
over my head.  I ask them to observe well, because once the donut
comes free there's only a split second before it hits the fence, or a
tree, or a person!  Of course it flies off in a straight line:
Newton's first law strikes again.  Then I ask them to think of
anything else that moves in a circle.  Sometimes I have to hint "in
space", but they can guess Earth around the Sun, or the Moon around
the Earth.  So that's the proof that there is a force keeping the Moon
around the Earth: if there were not, the Moon would fly off in a
straight line.  (This comes as a revelation to many adults and college
students...they were never helped to make the connection between real
life and Newton's abstract laws of motion.)  Then I ask them what the
name of that force is, and in each group at least one child knew it
was called gravity.

Third stage: we went to the sidewalk for the cart activity.  First,
each child gave a push to an unloaded cart, and then the same size
push to a cart loaded with 40 pounds of weights.  The heavier cart
accelerated much less.  This is the other facet of Newton's second
law: acceleration is proportional to force, but inversely proportional
to the mass of the object being accelerated.  The phrase "same size
push" is an attempt to make "same force" sound less technical.  Some
kids initially seemed to interpret it as "make the cart accelerate the
same amount" so I made sure to counter this by continually repeating
phrases like "use all the same muscles" or "push just as hard as last
time."  In cases where they still didn't quite get it, I asked if they
ever got so mad at their brother (or sister, or friend) that they
wanted to push them.  Yes, you can admit it!  Pretend the cart is your
brother, you are mad, and you push him.  Now, for the other cart,
you're still just as mad, so push just the same!

Also, a common misconception is to look at how far the cart travels as
a measure of the effect of the push.  We must not do this, because how
far it travels is a complicated function of how much friction there
is, whether it had to roll over a small stone or a crack, fight a gust
of wind, etc.  No, we must observe how fast the cart was moving
immediately after the push.

Finally, we get to Newton's third law.  I need two volunteers, one to
kneel on each cart.  Alone, each child can't get his or her cart to
start moving.  But they can if they push against each other, and this
results in equal and opposite accelerations if I wisely chose
volunteers of the similar mass.  This shows that forces come in equal
and opposite pairs, which is Newton's third law.  (The usual
formulation, "Every action has an equal and opposite reaction," is
very misleading because it gives the impression that the net result is
zero.) 

After giving each child a turn at this, we had a minute or two left in
some of the groups, so we did a more advanced third-law demo.  I got
on a cart and held a bathroom scale, while I gave the child on the
other cart a bathroom scale.  We pushed off each other's scales, and
her cart accelerated a lot while mine accelerated only a little.  I
had asked them to predict the accelerations, and they invariably get
that right.  But then I asked them what they thought the forces (the
readings on the scales as we pushed) were: more on my scale, more on
hers, or the same?  They invariably think more on mine, because I'm
bigger and so they think I must exert more force.  But the scale
readings are the same, which is just Newton's third law!  The effect
of the force (the acceleration) is different because she has little
mass and I have a lot, but the amount of force is the same.
Similarly, in a car collision, where the massive vehicle decelerates
relatively little while the light vehicle decelerates a lot; this can
only happen if the forces are the same!

The kids seemed to really like these activities.  In fact, Becca told
her mom so, and Becca is hard to impress.  The only thing I would do
differently is not buy such cheap bathroom scales.  They constantly
had to be re-zeroed, and were not very reliable or accurate.  But stay
away from the digital ones too, which you have to step on, step off,
step on again, etc.  These would be frustrating for kids in a pushing
experiment.