Dave jumps into the conversation several days later and shares the activity he uses in his physics class during the first day of school.

Subj: rolling balls
From: Dave

Hi Mike - that sounds great! I really like the idea of starting with a problem. Here's one I used this year.

I gave out sheets of paper with two curved, parallel lines on it - I think I got this originally from a Scientific American article on intuitive physics by McCloskey. I also gave them marbles, and said that the object is to try to get the marble to roll between the curved lines, without touching either line, and without touching the ball with your hand while it's between the lines.

A lot of students tried to get the ball moving in an arc of the right radius, then released the ball hoping it would continue to move in the arc. They'd try it a few times and find out that didn't work. A couple of students tried rolling the ball by putting a spin on it. They had trouble, but they actually managed to succeed one time when I was watching! (Of course, all of their attempts worked when I wasn't watching!) One group got a book, put the sheet of paper on the book, and tipped the cover a little so that the ball would be rolling on a slope. Another group got a ruler, and placed it between the lines so that, if the ball rolled along the ruler, it wouldn't touch either line. Then they discovered that the ruler wasn't necessary!

Eventually everybody figured out that if you just roll the ball without any tricks, it goes in a straight line. We talked about what everybody thinks in this situation that makes the problem tricky, and a couple of students talked about how you think that the ball will keep curving. We did another problem also (about wrapping a cord around the earth nice and snug - how much longer would the cord have to be if you wanted it propped up on bricks all around the earth - people always think it would have to be a lot longer.) and we had a discussion about how what you first think about something isn't always the way it is. I used that to tell them a little about what the course would be about.

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