Teaching Physics with the Physics Suite

Edward F. Redish

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Stopping a train

A toy train of mass m comes off a hill traveling at a velocity v0, rolls down an incline of height h, rolls a short distance on a straight track, and strikes a bumper containing a spring of spring constant k. The distances are as indicated on the figure below. The train is just rolling, not powered.

  1. Assuming that friction and the rotational energy of the wheels can be ignored, describe the changes in the forms of energy of the system starting with the instant the train begins down the hill until it comes to a stop at the bumper.
  2. What is the speed of the train, v, when it is on the straight piece of track? Express your answer in terms of the symbols given above
Note to the instructor: This problem is stated as it is in order to address three issues. 1. Students often fail to think about the time structure of a problem, and don't have a "mental movie" where they can watch what's happening and think about what's happening to the various physical variables. 2. Students sometimes focus on the changes in each kind of energy but don't thing about the fact that the changes are transformations from one kind to another and the changes compensate, leaving the totals the same. 3. Students often are uncomfortable with symbols and want to put in numbers right away. When they do this, they fail to learn to work with symbols. By not giving any numbers we are requiring them to begin to develop a valuable skill.

Page last modified November 13, 2004: PE25