Discrepant events (Blogging from the AAPT)

by Stephanie Chasteen on July 28, 2009

This session is about how using discrepant (or “surprising”) events to teach physics

There’s quite a bit of evidence showing that students don’t really get what we want them to get from demonstrations, but they do like them.  They get a lot more out of them if we ask them to predict the results of the demonstration in advance.  The Detroit area physics teachers went a step further and gave a popular session on “discrepant events” — using demonstrations with surprising results and asking students to first predict what will happen.  He often phrases the questions as mutliple choice and students vote with a show of hands (though clickers could be a great way to do this too).  You can find these at dmapt.org. Here are some examples:

Hoop and a disk

When you roll a hoop and a disk down a ramp, which one will win?  It’s the disk, because it’s harder to get going because it has a higher moment of inertia.  He had a few variations on this — a disk and a sphere, same mass (sphere wins), or a large sphere (2M) and a small sphere (M) (both roll at the same rate).


When you hang two masses from a bar using springs, and let one mass bounce, what will happen to the other mass?  (they resonate, so the 2nd mass starts to bounce and the 1st mass slows.  What if we change the 2nd mass so it’s not the same as the first mass?  We don’t get resonance in that case.  What if we use 2M on one spring, and make the 1st spring twice as long?  We get resonance again!

Projectile motion

We manage to hit one of those little troll dolls (a “conTROLLed experiment?”) with a ball launched from a little ball launcher.  If he changes the angle, will he still hit the doll?  Well… he cheated… the ball launcher has an angle-o-meter on the back, and he used comlementary angles (eg., 60/30) to manage to hit the target even with the changed angle.  A good activity for the first day of class.

Bike tire

Suspend a bike wheel from a string and get it spinning vertically.  What will happen when you let it go?  (It keeps spinning and precesses).  This one’s hard to describe in words…


Fran July 29, 2009 at 12:14 am

angle-o-meter. snrk! 😉

Shawn September 2, 2009 at 7:06 pm

My favorite discrepant events are those that surprise me, not just my students. I am getting more and more tired of the know-it-all attitude of teachers, including of myself, as I see how damaging it is to the risk-taking inquiry of the students.

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