RMP for Long Playing records. -- Rotations or Revolutions?
Does RPM (e.g., as used to describe the spinning of long playing records) stand for "rotations per minute" or "revolutions per minute"? I have the impression most people think it is revolutions per minute, but is the LP actually "revolving"?
Doesn't revolving refer to orbital motion, as in the moon revolves around the earth? Whereas, rotation refers to something spinning on its axis. With this understanding, isn't the LP rotating, and shouldn't it be "rotations per minute" unequivocally?
In other words, rotation and revolution are not synonyms.
By the way, I came across this self-inconsistent statement in Wikipedia:
"Revolutions per minute (abbreviated rpm, RPM, r/min, or r·min−1) is a unit of frequency: the number of full _rotations_ completed in one minute around a fixed axis. It is most commonly used as a measure of rotational speed or angular velocity of some mechanical component."
I think Dr. Bob pretty much summed it up. However, the revolution/rotation dichotomy is not foreign to the field of mechanics, and mechanical systems certainly exist in which the distinction is important.
In fact, this question came about as I was reading though a mechanical description of a "planetary gear" mechanism, having a central sun gear, a set of planetary gears, and an outer annular gear, all of which were meshed into a unit. In this description, it was intentionally noted that the sun gear "rotates" while the planetary gears (at times) both rotate on their own axes as well as revolve around the sun gear.
Therefore, to imply that the usage is properly different in mechanics than from astronomy I think is wrong. I think some people are just sloppy with usage.
Therefore, I am still puzzled as to why, in mechanics, there is a tendency not to be precise, and even in some cases to use the terms incorrectly, as in the "RPM" example of this question.
- Dr BobLv 61 decade agoFavorite Answer
Good point. If you had asked anyone what RPM stands for in the days of phonographs, they would have said "revolutions per minute."
Astronomers will often emphasize that the earth *rotates* on its axis, and *revolves* about the sun. The terminology used by astronomers is different from that used by mechanics. (For instance, a mechanic might speak of a lathe in terms of revolutions per minute.) The astronomical usage goes at least as far back as Copernicus, whose book on the motions of the planets has a Latin title that begins "De Revolutionibus." (But astronomers are describing a more complex situation. An engine has just one periodic motion, whereas a planet has two.)
Here's another example: Some people (including answers on this forum) will insist that the word "theory" must be used in a very precise way, and might give some other precise definition for "hypothesis" or "scientific law." But when you start looking at the literature, you find all sorts of inconsistencies in the way these terms are used.
It comes down to a matter of usage, and language is a far less precise system than mathematics. Scientists try to speak with precision, but it's not always easy.
If you look in a dictionary (e.g., Merriam-Webster Unabridged), you'll see that some of the definitions of "revolution" are synonymous with the word "rotation", and some of the definitions of "rotation" are synonymous with the word "revolution." This is one of those situations covered by the saying, "when in Rome, do as the Romans do." When you talk to astronomers (or take an astronomy course), use their terminology; when you talk to mechanics or engineers, use theirs.
Regarding your added remarks on planetary gears: I have to admit that I'm much more familiar with astronomical usage than mechanical usage. From what you say, it appears that when people have to describe two levels of turning, they use the same vocabulary as astronomers; but when they describe only one level, they get sloppy about terminology.
I came across this sentence in a web search:
"The cylinder rotation speeds are expressed in revolution per minute (rpm)."
The following site refers to the rotation of a wheel on an axle, and uses the term "revolution" to denote a 360-degree rotation.
This is similar to the previous quotation and seems to be a fairly common usage. "Rotation" means turning, and "revolution" means one full turn. This is similar to the line from Wikipedia mentioned in your question.
"One rpm (rotation per minute) is 1/60th of a revolution per second." (That's a real mixed usage.)
Answers.com says the following:
"While revolution is often used as a synonym for rotation, in many fields, particularly astronomy and related fields, revolution, often referred to as orbital revolution for clarity, is used when one body moves around another while rotation is used to mean the movement around an axis."
In summary, with the traditional usage by people outside of astronomy, it's reasonable to say that a record rotates at 33-and-one-third revolutions per minute.
- Tinman12Lv 61 decade ago
The LP's spun on the turn table at a rate of 33 1/3 rpms. R as in rotations.
You should bring this same question to the auto mechanics section for more fun. Mechanics always speak of engine speed in RPM's.
- Anonymous5 years ago
Yes I still have my LPs and 45s and I still play them. I still have my stereo so that I can play them whenever I want. The stereo is in a cabinet and does take up a lot of room,but I will not get rid of it because I would not be able to play my records. Nowadays modern players etc are more compact but they do not cater for 45s and LPs. I like nothing better then having what I call a dance and sing down memory lane with all my records.
- 1 decade ago
Revolution per Minute
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- science_joe_2000Lv 41 decade ago