Soup Can Races - Case Notes

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Author - John Shepherd - Fall 1997, Spring 2000

Background There are three soup cans. All are Campbell's soups. The gold can contains chicken broth and has a label weight of 298 grams; the silver can contains cream of mushroom and has a label weight of 305 grams; the red can contains chicken gumbo and has a label weight of 305 grams. The red can beats the silver can in paired competition and the gold can beats the red can.

The cans contain liquids of differing viscosity (the broth is least viscous, gumbo is in the middle, and cream of mushroom is the most viscous). The frictional drag between can and soup makes the more viscous gumbo rotate with the can, in a manner similar to a solid rock. The broth rotates less rapidly than the can because there is very little drag between can and broth, so this can roatates more in the manner of a hoop. Since momentum is conserved as rotational energy is transferred to the viscous soup, the can itself must slow down (or at least accelerate more slowly).

Teaching Notes The cans can be rolled down the ramp at the same time in a "race," or they can be rolled down the ramp one at a time and the elapsed time recorded from a stopwatch. I didn't let the studetns handle the cans. I ran the races as a demonstration. When you put the cans on the ramp, you need to put the cans against the edge of the ramp, with the can rim toward the center. This way the can will roll straight down the edge; if you turn the cans around, the will roll out into the middle of the ramp and it won't work very well.

In some classes, someone will hit on a solution right away. In my F97 class, all of the groups chose the hypothesis that said the heaviest can rolls the fastest. I asked them to design experiments to test their hypothesis (ie., to falsify it). They generally did a poor job of this. We had bowling balls and tractor trailers in some of their designs.

They also had difficulty in presenting a clean design to the rest of the class. I think they would have done this better later in the term.

Reference List Patrick Briggs & John Swez. 1995. Instructor’s Resource Manual to Accompany FW Bueche & DA Jerde. Principles of Physics. 6th ed. McGraw-Hill