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Feeling the force

September 30, 2010

My kids have no idea what a Newton is. And the little apples you can buy at AAPT don’t really help with this.

Me: Here, hold this fake apple. Now do you understand what a Newton is?

S: No. It’s a fake apple.

Me: But I got it at AAPT—it should teach you something!

So I’ve made a conscious effort to get kids to think more about how big their forces should be. One thing that really helps this is having the kids draw all their FBDs to scale and only do vector problems graphically. A couple of other quick strategies we’re trying

  • We spend some time getting students to think about their weight in Newtons, and playing around with measuring the weight of other objects in Newtons. Couple this with kids understanding that “the earth exerts a force of 10 N for every kilogram of mass of an object” (this is how they learn to think of it, and they pretty quickly see that their weight is hundreds of Newtons. They then think about this more—our weight isn’t enough to crush a person if you’re standing on them, but it is uncomfortable. Good, now we’re getting a useful scale for force—the pain scale.
  • Where kids really have trouble with this is the whole mosquito hitting a truck N3 deal. My kids, with lots of practice, eventually come to realize that the force of the truck on the mosquito is the same as the force of the mosquito on the truck, but this doesn’t give them any insight into how big the force is. So then I ask what does it take to crush a mosuqito? 100N, 10N, 1N? And suddenly kids are saying things like “I think a 100g mass would crush a mosquito, so the force must be less than 1N,” and “We were dropping coffee filters today, and they weight about 0.01N, I don’t think this would crush a mosquito, so it must be greater than 0.01 N.”
  • Finally, today, I thought of this question—I pick up a piece of paper and hit it. “How big was the force of my hand on the paper?” Again, the kids have no idea at first. But then I ask them to put the 500g mass on their knuckles. Did hitting the paper feel like this? No. And pretty soon, the figure out that it feels something like putting a ~50g mass on your knuckles, so the force must be around half a Newton. Score! And this leads to many more questions like why you can’t hit the paper harder, and why if you hit someone, it hurts much more.
  • I also am asking questions on assessments to test this—on my latest test, the students have to calculate the frictional force that holds a car up on a 19° incline (the slope of the world’s steepest road, in Dunedin, NZ), and then ask, “If this force disappeared, could you supply the same force?”

Of course, all of this would be way easier if we would just switch over to the metric system, but until that day, I’ll keep getting kids to hit pieces of paper.

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