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Friday, September 25, 2015

The First Law of Motion

My son and I were walking back from the library where he had just spent two straight hours playing his favorite video game (and was consequently in a mellow mood). 

As we marched on the pavement, we talked at first of this and that, chit-chat. Then it occurred to me this could be a good chance to strike up a conversation about something they'd be touching on at school soon.

"This guy, Newton," I started. "So he has three laws of motion, right?" 

"Right," he said, a little cautiously, but not too perturbed. He is used to my habit of starting conversations like this rather abruptly.

"Do you happen to know what they are?" 

He seemed a little dubious, so I backed up and decided to go even slower.

"Ok, let's talk about just the first one. It basically says objects that are at rest don't move, and if they are moving at a constant speed, they stay that way unless something acts on them to change their motion."

He nodded, this seemed familiar. "Because of their momentum," he noted.

I had noticed his laces were untied and pointed it out to him. We stopped by a fence while he put up his shoes to fix them.



"Yes, momentum, or..." I got a little confused myself, watching him. He was in fact right, but I wasn't sure in the moment if momentum was the right word to use about this law, so I said, "well, I think Inertia is the word we want. We say the objects have inertia, inertia of rest or inertia of motion."

I went on: "Do you know the ancient Greeks had their own version of this law, and it was rather different?"

He looked up, interested. We have been watching a very entertaining history TV show these days and the Groovy Greeks, he knows, are always an interesting lot.

"They thought objects prefer to be always in motion unless something acts on them to stop them."

He chewed on this, as I went on to say,  "But we know this isn't true. If you did this experiment in zero gravity and in a vacuum, for example, an object could stay still with nothing touching or acting on it."

Digressing here a bit, he said, "Oh, you know, my science teacher once got to experience zero-gravity. She said it was not a pleasant experience. Your insides feel like they're not supported."

"Hmm...," I said, "very interesting. I hadn't thought of how it would feel." 

He ran off at this point ahead of me. I took it as a signal he had gotten bored of the discussion we were having about Newton, but he smiled and clarified he didn't want to miss the chance to catch the light, so he had run ahead to push the button on time. Indeed, the walk signal showed up right away and we could cross. 

Seeing that I still had his attention, I continued as we crossed the road, "So, to them, the Greeks, every object they saw preferred to move and would only stop if prevented from moving. In particular they noticed objects always want to fall down, that is to say, move downwards, unless they were held up."

I gestured with my hands to illustrate this. We walked past some tall grasses, then a big box intended to collect clothing donations.

He thought about it a bit and remarked perceptively: "I see how what they were thinking makes a lot of sense. Imagine a rock at the very edge of a sheer cliff. It wouldn't even want to stay at the ground at the top of the cliff, it would rather fall even further down if it could. So the cliff is holding it in place."

"Right."

He nodded, chewing it further in his head.

I then asked, "Do you think the Greeks were wrong?"

He said "yes," right away.

I clarified a bit further: "I mean, if you think about the two different theories, they do agree on one point, that an object in motion could be stopped by a force acting on it. But they actually disagree about what would happen if an object was not moving and nothing was touching it or pulling/pushing at it. The Greek version says that is simply not possible, while Newton's law says it would stay still."

I continued, "the Greek version of this law applied to everything they encountered in their daily lives. We simply don't see such an object in our everyday experience. So for what we come across in our every day experience, both Newton's theory and the Greek theory are valid."

He responded, "I see. I kinda sympathize with them, they simply didn't know any better." 

"Exactly!" I said, "given what they were able to observe, their explanation, their scientific theory was adequate and consistent. In fact, perhaps it was even more satisfying because it doesn't invoke something intangible called a 'Force', the objects in their world-view are held back from moving by other objects such as the ground or the fluid air."

I changed gears a little, "Similarly, Newton's theory was adequate for his time, but in fact now we know even he was wrong. His theory offers a good approximation but is not quite right."

He seemed surprised. "I had no idea! Newton was wrong? But he's so famous!"

I said, "well, Einstein's (general) theory contradicts Newton. Newton believed Gravity was a direct interaction between the planet and objects like the apple. But Einstein didn't believe in what he called 'spooky action at a distance' ."

"Then why do they teach kids about Newton's Physics?"

"Because he wasn't completely wrong. His physics provides a pretty good approximation of most phenomena we deal with. And it would be too hard to teach kids in school about Einstein's theories."

He laughed, no doubt at the silliness of kids being taught something wrong on purpose in schools everywhere because the truth would be harder to explain.

"Basically, unlike Newton, who saw  saw  Gravity as a Force, Einstein saw it as resulting from the curvature of space-time."

We were home. He rested, tired, on the couch, but he was still looking at me, clearly confused by this complicated expression "curvature of space-time". 

"A somewhat intuitive analogy or model of what Einstein was saying is to think of the planet as a bowling ball on a piece of cloth, and any object near it such as a smaller ball will fall in towards the bowling ball because the fabric will have been reshaped in a way to make that happen. Einstein's point was that the planet doesn't call out to an apple at a distance to exert a force on it, just as the Bowling ball doesn't 'talk' to the smaller ball, rather the planet's mass distorts space and time in such a way that the natural motion of the ball leads it towards the planet."

"Ah! Remember that YouTube video we saw? They also showed how the smaller ball would loop round and round around the bowling ball if it was moving at a speed initially."


"Right. I remember it, that was a cool video!"

We both stopped talking, thinking in parallel, perhaps, about that shared memory.
Pleased, but thinking now of other things, he leaped from the couch, glided across the room, bounded up the stairs, and vanished.

*** 

His mother told me later he had narrated to her upstairs how Newton was wrong and told her excitedly about the bowling ball model of Einstein's theory of gravity... I am so glad our conversation had stirred something in him that he had wanted to share with others. 


P.S.: My conversation with my son was inspired by an excellent article by Arthur Steiner titled "The Story of Force: from Aristotle to Einstein". As the author writes, "a history-based exposure to the conceptual development of Newtonian mechanics is superior to a conventional textbook-centered approach, because it is contextual, shows the intellectual struggle involved in scientific thinking and relates better to students' knowledge and experience." I couldn't agree more.

Credits: images from Wikimedia; thanks to Sean C. for pointing me at the video by Feynman.  

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