# This Object ALWAYS Spins Counter-Clockwise, why?

## Main Question or Discussion Point

Hey guys,

I've read these forums a ton over the years and found everything I needed merely by searching. But alas, the day has come where I've needed to create an account to help me with a problem.

I have to determine why this object ALWAYS spins counter-clockwise. Whether you spin it clockwise, tap it, push it - it doesn't matter... pretty bizarre.

Here are some picture and appreciate the help:

## Answers and Replies

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berkeman
Mentor
Hey guys,

I've read these forums a ton over the years and found everything I needed merely by searching. But alas, the day has come where I've needed to create an account to help me with a problem.

I have to determine why this object ALWAYS spins counter-clockwise. Whether you spin it clockwise, tap it, push it - it doesn't matter... pretty bizarre.

Here are some picture and appreciate the help:

What you said kind of doesn't make sense. When you spin it clockwise it spins counterclockwise?

And can you give more details about what it is? Where did you get it?

Mech_Engineer
Gold Member

In Wales, it is called a rebellious celt. To us, it's the rattleback. "It" is a long, thin plastic toy with a base shaped like the hull of a boat. When you spin it one way, it turns a few times before the ends start to rattle up and down. The more it wobbles, the slower it rotates - until it stops spinning altogether. Finally, it starts to spin in the opposite direction. What could possibly cause this?

The first attempt to analyze rattlebacks was around a century ago. In the mid-1980s, two detailed mathematical analyses were done: one by Hermann Bondi (then Master of Churchill College, Cambridge) and the other by Mont Hubbard (Professor of Mechanical Engineering at the University of California, Davis).

D H
Staff Emeritus