June 23, 2013
June 23, 2013
June 26, 2013
23.1079.1 - 23.1079.10
Soup Can Races: Teaching Rotational Dynamics Energy-based SolutionsGiven our college’s urban student population, our students have little exposure to mechanicalsystems before they take engineering mechanics courses. Classroom demonstrations have beenfound to strongly support student learning and retention of conceptual ideas.The following paper outlines a course lecture based on a demonstration titled “Soup Can Races”in which students are asked to predict the order of finish of various soup cans that are raced orrolled down an inclined plane. To aid their decision making students are reminded of the basicenergy equation for the given system stating that the change in potential energy is equal to thesum of the translational kinetic energy and the rotational kinetic energy as well as the moment ofinertia for a solid cylinder and hoop about the axis of symmetry.The soup cans the students are asked to rank including their masses are as follows: Crème ofConcrete (622 g), Empty Can (51 g), Shell of Can which does not include the can ends (40 g),Tomato Soup (354 g), and Chicken Broth (347 g).The cans are then raced in front of a loud and very engaged audience followed by a discussion ofthe results. Typically, the discussion begins with the last place can, Shell of Can, which can bevery closely modeled as a hoop. The velocity of the can as a function of the change in height isderived using an energy-based solution. The result is shown to be independent of total mass ofthe hoop. This is demonstrated by racing the shell of can versus a larger and more massive hoopresulting in a tie.Secondly, the Crème of Concrete can is analyzed and compared to a solid cylinder using asimilar energy based analysis. The final velocity is again shown to be independent of total massthrough the derivation of the velocity of the can and demonstration in class. The velocity of arolling object released from rest on an inclined plane is presented to only be a function of thegeometric mass distribution of the object about the axis of rotation and independent of the totalmass.The Empty Can and Tomato Soup are then analyzed as more complex systems where therotational energy can be broken into the components. In the case of the Empty Can, the end ofthe cans and the shell of the can have different rotational energy contributions to the energybalance because the moment of inertia of the solid cylinder or thin disk and hoop are different.Finally, the remaining question, “Why is Chicken Broth the runaway winner?” can be answeredbased on the previous discussion assuming that in an ideal case the bulk of the liquid in the candoes rotate.
Bankhead, R. B. (2013, June), Soup Can Races: Teaching Rotational Dynamics Energy-based Solutions Paper presented at 2013 ASEE Annual Conference & Exposition, Atlanta, Georgia. https://peer.asee.org/22464
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