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From the archives: The Force, not midi-chlorians

July 20, 2011

While I’m on vacation, I’ve decided to repost some of my older posts from my early days in blogging, in the hopes that they might get a bit more discussion than before.

My kids love the force of air resistance. They draw it on everything. I mean they think an arthritic turtle must feel a force of air resistance. This becomes a problem for them really understanding Newton’s Laws since they think you always need some “propulsion force to keep an object going.” One way I tried to address this misconception this year was drawing on Rhett’s awesome “Flying R2D2—you’re doing it wrong post.”

I also asked them to try to get a sense of the force of the air acting on an object with some guiding questions.

  • Do you feel air resistance when you are sitting still? Walking? Running? Curiously most students said yes here)
  • Ok, what about swimming, you swim slower than you run, so why are swimmers so worried about drag?
  • Do runners need to shave to get faster? Why not?
  • When do you feel air resistance? Putting your hand out the window? How fast must you be going? 10 mi/h? 20? 40?
  • So does the air puck in our classroom feel a significant force of air resistance? What about the dynamics carts?
  • How do we determine if the force of air resistance is significant?

Then we went back to FBD pictionary, and looked at a short clip of a video of rocketman flying at constant velocity. I tell them again, rocket man is probably only going about 5 mi/h, and get them to look closely at the engine orientation.

Finally, they get the FBD. Two forces, the gravitational force of the earth on rocket man, and the contact force of the exhaust gases on rocketman. Equal forces, acting in opposite directions. Zero net force, constant velocity.

Then we look at R2, and have a great time talking about R2 flying through the clone factory on the sith lord home planet (I’m sure I just butchered SW Canon there), and that maybe even the factory operates under vacuum. I asked the kids what was wrong with the picture. It took some effort, but after a number of questions, and naming of forces, the students realized that only way for two forces to add to zero is if they are opposite one another, and since R2’s thrusters are angled, this isn’t possible, and so R2 can’t be moving at constant velocity. But Rhett already measured the velocity (I really wish I had ripped the scene of this from the DVD—I’ll do this next time), and analyzed it. It is constant velocity (an other curious observation, a couple of my kids looked at Rhett’s graph and said it’s almost constant velocity—I’m still trying to figure out how to get them to let go of the desire for perfect data—as if somehow, the only way it can be constant velocity is if the points lie dead on along the line).

Looks like constant velocity to me.

So once everyone got it, I asked the following question. “Surely it cost 500K to make this clip. Why did ILM screw up the physics?” This was a great chance to reinforce just how un-intuitive Newton’s laws can be. Together we explored 2 ideas.

1. No doubt ILM engineers studied and aced physics. However, they might have just been doing it for the tests, and still had the same misconceptions about how things move that Aristotle did. And so when given a change to animate R2, they don’t really think about FBDs and Newton’s 1st law. They just animate what they think it should look like.

2. ILM engineers are physics rock stars, and they know perfectly well how it should look, and their animation software probably has many laws of physics cooked-in. But they also know most of their audience don’t understand physics deeply, and so would expect an object like R2 to feel a force in the direction he’s traveling.

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