The Old Gate: A Lesson in Kinematics

Today my 3-year-old and my 4-year-old had physics class.

This isn’t remarkable– as children who are not yet in the concrete operational stage, they are constantly in motion, and that is the best practical illustration of physics one could possibly hope to have– but what was remarkable was the subject of today’s lesson.

I watched them play well into the dusk, until they couldn’t see their feet in front of them in the fading light.  First, Ithilien would find a rock about the size of a mango.  Then they would both climb up to the first terrace in our mountainside back yard, and Númenor would swing the old stick gate– a remnant, we think, of someone’s goat pen– into the “closed” position and hold it still.  The latch is broken, so something must hold it still, otherwise the natural slope of the yard swings the gate out over the wooden deck on the level below until its post stops it and it comes to rest, looking like the top of a Dutch door in a nonexistent wall.

Next, Ithilien would place the rock on top of the gate, balancing it carefully.  Finally, Númenor would give the gate a little push to send it on its way, and it would swing wide over the deck and be abruptly stopped by its post, whereupon the rock would be jarred off the top of the gate and continue forward and downward to the ground.  Both children positively screamed with laughter every time this happened, but eventually it became predictable– after perhaps two dozen trials, they began to vary the number and placement of the rocks.

I finally gave them notice that it was too dark to keep playing outside– one more trial and then they had to come in– and we talked about what they learned.

N: “We pushed the gate, then it stopped.  The air pushed the rock and made it jump.”

Me: “The air pushed it?  I don’t think so…”

N: “Actually, it is called in-ur-sha.  That is a French word for ‘it keeps going’.”

Me: “Yes, the rock did fall off the gate because of inertia, which is a Latin word meaning “lazy” or “inactive.”  Why didn’t the gate keep moving?”

I: “The gate– it hit the fence– and it just stopped– like this!” (mimes a cartoonish sudden stop and resulting vibration with hands)

Me: “That’s exactly right.  The gate hit the rest of the fence, and that stopped it, but nothing stopped the rock, so it kept going and fell off the gate.”

N:  “The rock only fell from the front from the gate.  It fell in front all ninety-eight times!” [sic erat dictum, but I think it was more like 50 times]

Me: “Yeah, that’s what I would guess– the rock kept moving the same way the gate swung.  What happened when the rock was close to the hinges?”

I: “When the rock– it was close to the hinges– and it did not fall off!  And Númenor pushed the gate– and then the rock– it did fall!”

Me: “Exactly!  When the rock was close to the hinges, the gate swinging by itself didn’t give it enough inertia to overcome friction, so it did not fall off the gate until you swung the gate with more force.”

N:  “Yeah!  It had friction because of the wood– it is not smooth, it is all scratchy.”

Me:  “Uh-huh.  And the rock is probably bumpy, too, and that adds friction.  What about when there were two rocks on the gate?”

N: “One on the end of the gate did fall, but one by the hinges did not fall.  It had too many friction and not enough pushing, because it was closer.”

Me: “They probably had basically the same friction, but the one by the hinges was not acted upon by sufficient force.”

I: “The rock– when it fall– it maked a big noise like CRACCCCKK!”

N: “When Ithilien pushed the gate like this– ” (mimes pushing hard) “– the rock made a big noise.  When I pushed it like this–” (mimes a tiny push) “– it made a same sound.”

Me: “What do you think that means?”

N: “Um.  I don’t know.  Maybe the rock falled the same?”

Me: “It always fell from the top of the gate, so it fell the same distance regardless of how hard you pushed the gate sideways.”

I: “Gravity is how things fall down!”

Me: “Right.  And gravity was the only force pulling down on the rock.  So the rock would have the same speed hitting the ground no matter how hard you push the gate.”

N: “Yeah.  Gravity is how things are pulled by heavier things.”

Me: “Hmm.  Well, technically it’s how things are pulled by more massive things.”

N: “More massive, yeah.  Like Earth pulled on us’s rock?”

Me: “Exactly.”

And there it is.  In less than two hours, left basically unattended with derelict farm infrastructure and rocks, my three and four year old children discovered that vertical and horizontal forces are independent, that forces on the end of a lever are amplified relative to forces at the fulcrum, and that inertia can be overcome by additional force.  They also reviewed gravity, inertia, and friction, which are concepts we’ve talked about (and they’ve seen on The Magic School Bus) before.  They made hypotheses and collected data and verbalized the significance of their results.

Which is to say nothing of all they learned about what lives under rocks and inhabits leaf piles on warm October evenings.