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I am in the middle of a discussion with a friend about the meaning of the term "clockwise".

Wikipedia indicates that a clockwise rotation goes as top-right-down-left.

However, my friend argues that "the clock is facing opposite to you. So if you rotate top-right-down-left facing the side the clock is facing then it is anti-clockwise."

Our question, therefore, is

does "clockwise" mean top-right-down-left or left-down-right-top?

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closed as off-topic by Dilaton, Chris White, Dan, Waffle's Crazy Peanut, David Z Jul 21 '13 at 19:55

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Please add reason for downvote! –  Madhu Jul 18 '13 at 6:01
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This question appears to be off-topic because it is about English terminology having nothing to do with physics. –  Chris White Jul 18 '13 at 15:42
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I agree with Chris White. Conventions aren't physics. It is just English. But I won't downvote... ( P.S. This version (v4) is much better than the previous version, which may be the reason for me not to downvote. ) –  Dimensio1n0 Jul 19 '13 at 15:34
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I disagree. "Clockwise" is an important, ubiquitous technical term in the physical literature, and it is perfectly fine to have questions about their specific meaning and the conventions that surround such technical terms. Conventions are not physical insight but they are a crucial part of transmitting and understanding it. –  Emilio Pisanty Jul 19 '13 at 15:40
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Meta discussion. –  dmckee Jul 22 '13 at 22:01

3 Answers 3

up vote 8 down vote accepted

The clockwise direction is normally defined by the right hand grip rule.

Clockwise

When your thumb is pointing away from you, your fingers are curled clockwise. So when you look at a clock the axis of rotation is away from you through the clock.

I'd guess the downvotes are because people believe your question is not physics related, but in fact this rule is how you determine the direction of the angular momentum vector, so there is a connection with physics.

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Thanks for your response! Do you have any comment on my friends theory? The clock is actually facing opposite to you. So when you rotate your finger in TOP-RIGHT-DOWN-LEFT direction as clock's needle its anti-clockwise. One other similar example is when you shake your hands to opposite person, you shake you right hand to other person's right hand not the left hand. –  Madhu Jul 18 '13 at 6:52
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A down vote? What on Earth was that for? This answer seems to me thoroughly uncontroversial. –  John Rennie Jul 19 '13 at 15:07
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After spending far too much time on this, because I assumed you were after something deeper, which I attempted to understand and check, I think that your answer is only simply adding confusion. You are replacing one simple object (the clock) that has a single way to be looked at by another (the hand) that exists in two chiral variants and has no natural viewing angle (more accurately, the viewing angle you use is not the natural one). You confused @Emilio, and you confused me even more. Explanation should reduce to simpler concepts, not to more complex ones. –  babou Jul 21 '13 at 17:50
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@babou Actually, it is possible to unambiguously define right-handedness in terms of observations of elementary particle decays. It is however, easier just to assume a standard human who know his right from left unless circumstances really prevent that. –  dmckee Jul 21 '13 at 20:20
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There are people who are unable to tell their right hand from their left. (They do exist, I've met some of them.) How would you explain the right-hand rule to them? –  Eugene Seidel Jul 28 '13 at 20:07

Your confusion arises because the term (anti)clockwise, when used by itself, is ambiguous, and should always be used with a statement like "as seen from the top" (unless that is absolutely obvious$^1$). The reason for this is that "clockwise" defines a direction of rotation within a plane, but does not specify which side the plane is observed from. (In more technical language, "clockwise" defines an order for two basis vectors for the plane, but does not specify a sign for the normal.) If you take a transparent clock mounted on a glass window, it will rotate clockwise or anticlockwise depending on what side of the window you're standing.

That said, a statement like "clockwise as seen from the top" is not ambiguous. This refers to the direction of rotation of the hands of a (normal, non-transparent) clock as you observe them. It coincides with the direction your fingers point to when you put your left hand in front of you with your thumb pointing towards you. In your terms,

clockwise means "top-right-down-left".

Additionally, if the question is purely about a planar problem (i.e. when doing 2D analytical geometry), then there is a canonical way to understand the term. Put the page on the wall so you can read it, next to a clock whose face you can see; its hands will move clockwise. This is equivalent to setting the plane normal "up", towards you. Put your left hand on the page with your thumb toward you, and your fingers will be clockwise.

It must also be noted that the "positive" direction of rotation is usually defined to be anticlockwise. Given a directed axis, put your right hand with your thumb pointing along the axis in the given direction, and then your fingers will point to the "positive" direction of rotation.

$^1$One example of this is hurricanes, which are said to rotate (anti)clockwise in the (northern) southern hemisphere. Here of course you're supposed to look at them from above (e.g. from space). If you see one from the ground (preferably from the eye, and from inside a well-protected building) the rotation will be inverted.

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Apparently, there seems to be no agreed upon definition of clockwise in 3D space - but an a clear agreement regarding the meaning in a plane "seen from above"

As I am starting to answer, I have no idea what is the correct answer, if any. It is the "if any" that gets me started on this topic. I am certainly not an expert, I never could tell my left from my right, especially after having physics classes, but I am looking for the available evidence.

To be honest I got actually very confused by John Rennie's answers, because I did not see the need to introduce yet another device, which you have to orient like the first, but not so naturally ... At least there is only one kind of clocks (except for funny souvenirs), while there are two kinds of hands to add to confusion. Also there is only one natural way to look at a clock, while there is no specific or normalized way to look at your hand, though having the thumb first is the natural position ... !

The word "clockwise" is used more than 800 times on physics.stackexchange.com according to the very approximative figures of google (I used: clockwise site:physics.stackexchange.com). This adverb must have some relevance to physics.

When searching the web, or wikipedia, for a formal definition of (counter-)clockwise in 3D, there seems to be none to be found. the answer by @Emilio is close to mine as he does seem to consider that the adverb "clockwise" has a well defined meaning without further precision.

The Internet being somewhat too large a place for me, I looked at the uses of this adverb on physics.stackexchange.com. I even limited myself to the first twenty answers from Google, as this takes considerable time. Be my guest to do more.

Many uses are simply informal. Wich way does this or that turn when you (un)screw or look at its rotation (like a clock). Bicycle pedals is a good example. In many other cases, it is only used to indicate changes in the sign of angular velocity, with corresponding changes in the sign of other quantities, without any precise chiral information. Note, for example, qthat when talking of hurricanes, it is always rotation as seen from above, like a clock seen from the front.

In Vector Nature Of Angular Velocity, the use is clearly a 2D planar use, about a plane seen from above. The vector considered does follow the right-hand rule. But it is not construed as a definition of clock-wise rotation in 3-D.

The same is true in Alkali atom in oscilating electromagnetic field. Indeed, it talks of counter-clockwise rotation as it look at the right hand from the end of the thumb.

In the answer to question What is negative angular acceleration?, things are a bit confused as it mentions the right hand rule, but seems to describe the opposite. Then it also defines clockwise as the positive rotation, while it is usually considered negative in mathematics (well, it did in my school times, but I would not know the current fads). Again, this is not construed as a definition of clockwise in 3D.

Question Lenz' law versus $-\frac{d\Omega}{dt}$ uses "clockwise (using right-hand notation)". The answer too makes explicit reference to the right hand rule to define clockwise.

One answer to Applying the right-hand rule for magnetic forces explicitly uses the right hand rule, and explains that it is useful because the direction is "counter-clockwise or clockwise depending on what side of the plane you are looking at".

One thing in common to all the uses where it matters is that the authors do not seem to think that there is an agreed upon definition of (counter-)clockwise in 3D which they can rely on for unambiguous communication.

The other thing in common (except maybe for the third) is that seem all to understand clockwise as "like a clock seen from above", when you use the right hand rule to translate what is being said.

Based of this evidence, admittedly needing more investigation,
my own temporary conclusion is that, unless someone can point to an "official" and agreed upon definition of the concept of clockwise rotation in 3D, it is probably inappropriate to assume such a definition.

The other conclusion is that there is clear consensus that clockwise means, in a plane, like a clock arms in the same plane assuming the clock is facing you.

An interesting aspect of it is that the word (counter-)clockwise seems very much used because it has intuitive meaning, apparently more than the right-hand rule. But it has sometimes to be defined more precisely, by the right-hand rule or equivalent means (you look at a clock in the XY plane from the positive Z ...).

Another clear conclusion, at least to me, is that it is an appropriate question, and it should be upvoted. It may not be physics in the most basic sense, but it is obviously a tool for the physicist, and without proper tools technicians cannot work.

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@JohnRennie Sorry for the error. The problem is that other answers are hidden when I edit, so that I have to rely on my deficient memory. –  babou Jul 21 '13 at 20:20
    
This is not entirely true. Experiments on the decay of neutral kaons (to take one instance) can be analyzed to provide both an absolute definition of anit- versus normal-matter and an absolute definition of right- versus left-handedness. From there we simple note that clockwise is "into the wall" by the right-hand rule. (Historically of course we established these things by reference to ourselves, but we can now communicate both ideas to a distance correspondent who we can not meet, and they will know what we mean despite not being able to check the original referents). –  dmckee Jul 21 '13 at 20:25
    
@dm All I said is that clockwise rotation has no well defined meaning in 3D, and the right had rule is really the only useful concept. I never said there is no absolute definition of handedness. I think the "explanation" by John Rennie introduced confusion in the issue discussed, and I first thought he was trying to generalize clockwiseness to a substitute for the left hand rule. So I check whether there was anything of the kind in what people write. I protested the use of hands rather than clock, not of a relation between the rotation of a clock (which is chiral) with other chiral definitions –  babou Jul 21 '13 at 21:29
    
@babou: at the time I posted the question was attracting a lot of downvotes, and the main aim was to show that it wasn't a completely silly question, there was some relation to physics, and hopefully stop further downvotes (in this I was successful! :-). With the comments and your answer the discussion has moved on to the question of handedness in physics, which is beyond what I saw as the scope of the question. –  John Rennie Jul 22 '13 at 5:55
    
@John Thanks. Actually I did not want to discuss chirality in physics. Dmckee misread me, as much as I misread you. I did not say a wrong word in that respect, afaik. I try to write carefully. I had the same purpose as you, and even upvoted OP. A very similar question physics.stackexchange.com/questions/2702 was treated decently. There is no reason it should have been different here. I did a real job of analyzing the use of chirality mnemonics, even though your answer had confused me, and all I got was a downvote for an unknown reason. Complete failure for all, moderators too. –  babou Jul 22 '13 at 23:01

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