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## Merry-Go-Round: Instructor's Guide

### Main Ideas

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.

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### Props/Equipment

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.

### Activity

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.

### Wrap-Up

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

### Extensions

• 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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