# RPM Question **URL:** \ **Category:** Factual Questions\ **Created:** [June 1, 2003, 10:43pm UTC](https://boards.straightdope.com/t/rpm-question/179013 "2003-06-01T22:43:25Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [June 1, 2003, 10:43pm UTC](https://boards.straightdope.com/t/rpm-question/179013/1 "2003-06-01T22:43:25Z")
I feel stupid for asking this, but I can’t seem to find the formula in any of the various books I have lying around at the moment, so I’m hoping someone can provide me with the formula. If I know the speed of the outer edge of a spinning disk, how do I go about finding the speed of a point closer to the center? Is it a simple ratio (i.e. if the disk is ten inches in diameter, and I’m trying to find the RPM of a point one inch from the center, is the speed of the point I’m trying to find 1/10th of the outer edge?) or is there a specific formula for this? ---
**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:** [June 1, 2003, 10:53pm UTC](https://boards.straightdope.com/t/rpm-question/179013/2 "2003-06-01T22:53:05Z")
The revolutions per minute of any point on the wheel will be the same as any other point, as long as the wheel is solid. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [June 1, 2003, 10:53pm UTC](https://boards.straightdope.com/t/rpm-question/179013/3 "2003-06-01T22:53:45Z")
The RPM of any point on the disk will be the same as any other point. To find the RPM of the disk, knowing the speed of rotation use RPM = V / (pi \* D) \* 60, where V is the velocity in units/second, and D is the diameter in the same linear units. Is this what you wanted? ---
**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:** [June 1, 2003, 11:00pm UTC](https://boards.straightdope.com/t/rpm-question/179013/4 "2003-06-01T23:00:00Z")
Oh wait, you’re talking about the rate of travel of a given point, aren’t you? Since pi appears on both sides of the equation, it cancels out; a point halfway between the centre and the rim describes a circumference which is half that of the rim and therefore it has half the distance to travle in a single revolution, so it will be travelling at half the speed of a point on the circumference. ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [June 1, 2003, 11:22pm UTC](https://boards.straightdope.com/t/rpm-question/179013/5 "2003-06-01T23:22:14Z")
That’s it exactly. Thanks. ---
**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:** [June 2, 2003, 6:12am UTC](https://boards.straightdope.com/t/rpm-question/179013/6 "2003-06-02T06:12:14Z")
Are you bulding a speedometer of some kind? ---
**Author:** ![Tuckerfan](https://avatars.discourse-cdn.com/v4/letter/t/eada6e/32.png) [@Tuckerfan](https://boards.straightdope.com/u/Tuckerfan)\ **Post date:** [June 2, 2003, 12:12pm UTC](https://boards.straightdope.com/t/rpm-question/179013/7 "2003-06-02T12:12:47Z")
> [@](#): > > \*Originally posted by Mangetout \* > \*\*Are you bulding a speedometer of some kind? \*\* [Steamer](http://boards.straightdope.com/sdmb/showthread.php?s=&postid=3464059). Of course, I _might_ need to build a speedo as well, depending upon how it all works out. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [June 2, 2003, 12:51pm UTC](https://boards.straightdope.com/t/rpm-question/179013/8 "2003-06-02T12:51:51Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > **The RPM of any point on the disk will be the same as any other point. To find the RPM of the disk, knowing the speed of rotation use RPM = V / (pi \* D) \* 60, where V is the velocity in units/second, and D is the diameter in the same linear units.** Agree. But I would add an extra set of parenthesis to keep the 60 in the numerator: RPM = **(V / (pi \* D))** \* 60 ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [June 2, 2003, 12:55pm UTC](https://boards.straightdope.com/t/rpm-question/179013/9 "2003-06-02T12:55:20Z")
Quite right. Thank you.