Purpose:Illustrate a tricky problem in rotational dynamics.
Description:A tricycle is "fixed" so that the steering wheel is locked in the forward/backward direction. When a rope is attached to the upper pedal (pictured at left), held parallel to the floor, and pulled, the tricycle clearly moves in the direction of the pull (the forward direction). (See this on an mpeg video by clicking on the picture at the left above.) Q: How will the tricycle move if the rope is attached to the lower pedal, as shown in the second photograph, and gently pulled? A: The tricycle will move forward, in the direction of the pull. This counterintuitive result can be argued qualitatively by viewing the system in the coordinate system of the wheel. Compare the pull with starting the tricycle by sitting on the seat and pulling the lower pedal backward.