# no movement in a spinning disk? **URL:** \ **Category:** Factual Questions\ **Created:** [December 29, 2010, 3:18pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704 "2010-12-29T15:18:06Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![dafox99](https://avatars.discourse-cdn.com/v4/letter/d/bbe5ce/32.png) [@dafox99](https://boards.straightdope.com/u/dafox99)\ **Post date:** [December 29, 2010, 3:18pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/1 "2010-12-29T15:18:06Z")
so, you have a spinning disk … like a record or CD. Draw a line through the middle to bisect it. Pick two points equidistant from the center and measure the instant velocities of both. They are equal in opposite directions. move close to the center and do it again. They are still opposite and equal … but slower. Do it again. So, when you are VERY VERY close to the center, you have two points moving in opposite directions very slowly. Soooo … doesn’t that mean when you move them on top of each other (you are in the middle), it’s not moving at all? The center of the spinning disk isn’t moving? ---
**Author:** ![Son\_of\_a\_Rich](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/son_of_a_rich/32/5439_2.png) [@Son\_of\_a\_Rich](https://boards.straightdope.com/u/Son_of_a_Rich)\ **Post date:** [December 29, 2010, 3:23pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/2 "2010-12-29T15:23:47Z")
This reminds me of a Calvin & Hobbes where Calvin’s uncle shows him the amazing properties of a spinning phonograph record. In the last panel, Calvin’s in bed, wide-eyed and unable to sleep. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 29, 2010, 3:24pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/3 "2010-12-29T15:24:17Z")
It means that the single point is not moving _relative to itself_, but you already know that, because it’s a tautology. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [December 29, 2010, 5:14pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/4 "2010-12-29T17:14:20Z")
The center point is rotating but not translating, correct. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [December 29, 2010, 5:16pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/5 "2010-12-29T17:16:52Z")
The story illustrates that angular rotation and linear velocity are not the same thing. It is true that the point at the center of the record has no linear velocity. However, it still has angular rotation. In fact, the linear velocity as things are spinning around can be calculated by multiplying the rotational speed by the distance from the center. At 0 distance (the center), you have 0 linear velocity… but no change in the rotational speed. Anyway, since the common English definition of “moving” would include both spinning around and moving forward, we should conclude that the center of the record is, in fact, moving. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 29, 2010, 5:38pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/6 "2010-12-29T17:38:29Z")
You are illustrating a version of [Zeno’s Paradox.](http://en.wikipedia.org/wiki/Zeno's_paradoxes) Using the same logic, it can be shown that if you jump off of a roof without a parachute, you never reach the ground. I wouldn’t advise trying to prove it. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [December 29, 2010, 5:40pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/7 "2010-12-29T17:40:23Z")
The center of an actual disk, being some kind of matter, is never dimensionless, so there is no ‘point’ that is part of the disc itself. And we tend to use phrases like ‘move the point’, but actually we mean that something else is located at a different point. So if its an actual ‘point’ that we are talking about, it isn’t moving. Every material part of the disk is moving though. (stupid nitpick, but Euclidean geometry is one the few areas of mathematics I know anything about). ---
**Author:** ![postcards](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@postcards](https://boards.straightdope.com/u/postcards)\ **Post date:** [December 29, 2010, 10:17pm UTC](https://boards.straightdope.com/t/no-movement-in-a-spinning-disk/565704/8 "2010-12-29T22:17:24Z")
> [@Son\_of\_a\_Rich](#): > > This reminds me of a Calvin & Hobbes where Calvin’s uncle shows him the amazing properties of a spinning phonograph record. In the last panel, Calvin’s in bed, wide-eyed and unable to sleep. Calvin’s dad, not uncle, and this was the first thing I thought of, too.