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?
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.
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?
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.
That’s it exactly. Thanks.
Are you bulding a speedometer of some kind?
Steamer. Of course, I might need to build a speedo as well, depending upon how it all works out.
Agree. But I would add an extra set of parenthesis to keep the 60 in the numerator:
RPM = (V / (pi * D)) * 60
Quite right. Thank you.