# A question for all you physicists.

Forum Editor 18:06 06 Dec 2009
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I enjoyed a pleasant lunch in the Oxfordshire countryside today, in the company of old friends. Christmas crackers were produced at the end of the meal, and three people got small spinning tops in theirs.

Here's the question - which nobody (including a nuclear physicist) could answer:-

Why, when you spin a top clockwise, does it always end up by making an anti clockwise rotation as it ends its spin, after it has fallen on its side? We tried all three tops a dozen times or so, and all of them made this anti-clockwise rotation. Thereb has to be a physics-based reason, but what is it?

octal 18:18 06 Dec 2009

Could be something to do with the Coriolis effect.

rdave13 18:27 06 Dec 2009

Centrifugal force, inertia and equilibrium. Spin the top anticlockwise and it will make a clockwise rotation just before it rests.

8-|

Wilham 19:01 06 Dec 2009

We live in 3 space with axes (say) x, y, z.
Angular momentum is conserved in the absence of any couple. After initial spin about vertical z axis air friction slows top, and while toppling, a couple is produced by gravity force and not-in-line reaction with table force.

This moving couple builds up equal turning power (moment) about both the x and y axes while on the move. The requirement is a high c of g position in the top and a low friction surface.

Unlike linear momentum, angular momentum is not three-way independent because each axis spin involves linear motion in the other two axes.

The energy to do this is gravitation, and with clever design it is possible to spin in the opposite direction upside down before it stops.

peter99co 19:09 06 Dec 2009

Except in Australia where it goes the other way?

rdave13 19:36 06 Dec 2009

Of course, so long as you spin it the other way to start with...then it will stop the opposite way you originally spin it at first whether you're in UK or Aussie... possibly. :(
Has to happen, it's a law of nature.

Legolas 19:43 06 Dec 2009

Just as I thought ;)))

DieSse 19:45 06 Dec 2009

When it falls on it's side spinning clockwise, all you have to do is picture the top as a wheel on a flat surface - it will move backwards from the direction of rotation at the point of contact, just like any wheel.

This makes the top move anti-clockwise.

Nothing to do with conservation of anything really - simple mechanics.

Picture it more easily - take a CD, and holding it flat turn it clockwise. Now tilt it forwards until the edge touches a flat surface, and keep turning it - it will want to move "backwards", which in the top will translate to anticlockwise.

Same in Australia by the way!

DieSse 19:48 06 Dec 2009

What actually causes it - friction, between the top "flat" part of the top, and the table.

DieSse 20:01 06 Dec 2009

.
How about some tough questions ;-))

BRYNIT 20:29 06 Dec 2009

I could be wrong but it could be due to Force. Reading through Newton,s second and third law although I have difficulty fully understanding it my self it may give the answer.