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The ultimate N3 test: tug of war

September 30, 2010

One of my most favorite blog posts on the internets is Frank’s Lesson Progression in Projectile Motion, perhaps because I used to use the very same stage 0 diagrams for years, and expected kids to just “see it”—I mean how hard is it to see that the x-component is constant velocity, and the y-component is constant acceleration. Yeah, right—somehow Aristotle and Galileo missed this one, but the 14 year olds in my class should have no trouble, right?

My more recent struggles have been more of what Dan Meyer points out as the breaking a real problem down into a thousand baby steps that rob the joy of the whole exercise. I mean I got a rope—a tug of war rope, and I used to give my kids this worksheet.

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Ok, kids, we can go play, as soon as you do questions 1-10. This is not a winning strategy for learning.

I’m glad to say that my experiences at modeling have convinced me that my students aren’t baby birds, and I don’t need to pre-digest the physics for them. So sort of on the fly, I told the students that they’d been hired as consultants by the National Tug of War League to advise them on the 2010 NTOWL draft. Let’s take 15 minutes for you to figure out what are the key points for the owners to keep in mind when assembling the best teams, and we set the timer for 15 minutes to write up whiteboards and deliver 3 minute presentations.

Along the way, I ask a few key questions:

  • Draw a FBD for each team, in terms of forces, what does it take for a team to win?
  • What does the FBD of the rope look like?
  • Is the winner the team that pulls harder on the rope?
  • Why do people always lean back to win TOW?
  • Are the forces on the rope equal because of N3?  (They laugh now—No way I’m falling for that one, Mr. B).

I made this keynote after the fact just to remind myself next year. Note to self, I really need a better team name and logo.

Presentations are awesome. Kids have great whiteboards and lots of questions. One group realizes that the rope’s net force is almost always zero, and this means both teams are pulling equally hard on the rope. Some kids realize that cleats dig into the ground, and give you a sideways normal force of the ground that can get big. Other teams realize that you winning team gets the ground to exert the largest force on it, and this means they have to push harder on the ground. Then they make specific recommendations—leg strength more important than arm strength (which can ‘t be neglected), mass is a factor (bigger is better for more inertia and larger frictional force).

All along the way NTOWL is growing—one kid is working up what stats you’d need for a fantasy NTOWL league, another is talking about what stadiums might look like, and what might be good halftime entertainment.

Now kids are psyched, and its time to put these ideas into action. I deicde to pick on two good natured guys and have them TOW without any shoes (socks only) against two of the smallest girls. It’s a rout! The girls crush the guys and everyone sees the power of the coefficient of friction (of course, they don’t know to call it that yet). Then we go outstide for NTOWL research and the kids have a blast trying out all sorts of scenarios: 1 team on brick vs 1 team on grass, tug of war uphill, tug of war against the tree (a losing proposition), and much more. All this took a little more than 50 mintues, and I think the students were far more engaged and learned much more than if I had spent 50 minutes forcing them to complete a worksheet and then gone outside with the rope.

Oh, and it turns out there’s an Tug of War International Federation. Who knew?

5 Comments leave one →
  1. September 30, 2010 7:15 pm

    This is awesome. It totally leads into friction and kids can design their own investigations to determine the factors affecting friction.

    I used TOW as a motivator for friction last year and it worked well. In the end, we had to determine who would win a TOW based on friction data for each teammate’s shoe.

    Which leads me to ask: What do you use for a rope? Where would I get one? (I am currently using a way too short rope for towing cars.)

    • September 30, 2010 7:22 pm

      Ooh, I love the shoes connection. Only problem is I introduce this before we really get down and dirty with the frictional force. But maybe we can do this as a follow up.

      I had to go around to all of the various divisions to find a rope—turns out that only the elementary school had one. It looks like you can find them for around $50 on the internets.

      What I really want is a mythbusters-like force scale that reads in kiloNewtons that I can use to measure the force of the kids trying to pull on the tree.

  2. October 4, 2010 2:03 am

    Great idea here. Just for the sake of making sure that we set good examples for kids (and not to be a jerk), I should point out that Aardvark is misspelled. Thanks for the post!

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