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Merry-Go-Round: Instructor's Guide

Main Ideas

Students' Task

Estimated Time: 10 minutes in class + 30–60 minutes outside class

Start the chair/platform going counterclockwise, sit in it, and attempt the following:

  • Roll a ball to a person standing beside the chair/table.
  • Roll a ball between your left and right hands in the radial direction.
  • Roll a ball between your left and right hands in the direction perpendicular to the radial direction.

Alternative task, requiring only a rotating whiteboard:

  • Mark a whiteboard with a rectangular coordinate grid, put it on a rotating platform, and, standing next to it, attempt to draw a straight line.

Predict/describe/sketch what happens in each case, both as seen by observers on the ground and by observers on the rotating platform.

WARNING: Students prone to dizziness should not be required to actually perform these tasks, but should instead observe other students.

Prerequisite Knowledge



This activity requires a merry-go-round!

A suitable alternative can be constructed; what is needed is a rotating chair with a flat desktop attached.
Another option is any mechanism which allows a whiteboard to spin smoothly.
If all else fails, show the video.


This activity should really be done outside of class, e.g. as homework, but it is well worth spending some class time on it to get students started — that is, if you have access to an in-house merry-go-round or equivalent.


  • Which way does the ball (appear to) curve?
  • Show the video. (Note that this merry-go-round goes clockwise.)


  • Is a boomerang possible?
  • Are there trajectories with multiple loops?
  • Are there trajectories which loop forever?
  • What happens on a curved rotating surface, such as the Earth?

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