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When we want to remove a cork from a bottle first we turn the cork. Turning in one direction makes it easier to remove in the axial direction.

Does anyone know something more about this?

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    $\begingroup$ Is this a fizzly champagne like content bottle? $\endgroup$ Jul 26 at 22:54
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    $\begingroup$ Could you please explain which cork type are you talking about? Normal wine cork does not stick out of the bottle to be able to grab it to twist it. The normal cork screw en.wikipedia.org/wiki/Corkscrew#/media/… does not allow you to to twist the cork. --- So normal wine corks are usually being removed just by a longitudinal pull force. $\endgroup$
    – pabouk
    Jul 27 at 8:49
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    $\begingroup$ @pabouk You can apply a twisting pressure to the cork without turning just the corkscrew by also applying a lateral pressure simultaneously (or putting the corkscrew in at a slight angle). I believe most anyone that's served wine at a restaurant for more than one night does this naturally without thought. $\endgroup$
    – TCooper
    Jul 27 at 21:54
  • $\begingroup$ @pabouk Sparkling wines, e.g., Champagne and Prosecco, have protruding corks, to which common corkscrews cannot be applied; typically you use twist and pull to open such types of wine. $\endgroup$
    – gboffi
    Jul 29 at 14:58
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When the cork is stuck and stationary, it is static friction which is culpable in keeping it fixed.

As soon as the cork moves - in any direction - the static friction is replaced by kinetic friction.

Kinetic friction, $f_k=\mu_k n$, is typically lower than the maximum static friction, $f_s\leq \mu_s n$ (because the kinetic friction coefficient typically is smaller than the static friction coefficient, $\mu_k<\mu_s$), and so, whenever you want to move something that is stuck, try to make it twist and turn and move before pulling it out.


With some downvoters and commentators bringing to my attention, that the answer above is not fully sufficient, I have below added the missing half covering the question of leverage.

Naturally, it is only a good trick to rotate the cork and then pull it out, if overcoming static friction to make it rotate is easier than overcoming static friction by pulling it straight out. As the comments mention, this can indeed be easier due to leverage:

  • Pulling it straight out requires the force, $F_{pull}$, exerted by your arm to match and overcome that of the static friction, $f_s$, fully, one-to-one. You are then fighting Newton's 1st law directly and must exert a force: $$\sum F > 0\quad\Leftrightarrow\quad F_{you}-f_s>0 \quad\Leftrightarrow\quad F_{pull}>f_s$$ There might other contributing factors to the necessary force as well, such as the pressure in the bottle as another answer points at.

  • Rotating the cork can be done by applying force at the far ends of the handle of the cork screw/wine opener tool. That force creates a torque, $\tau$, and the farther away, $r$, from the centre the force is applied (the greater the leverage), the greater does the torque become: $$\tau=F_{you}r_{handle}.$$ This torque in turn causes a shear force, $F_{cork}$, against the static friction forces at the cork perifery. As long as the tool handle allows for more leverage than the radius of the cork itself, $r_{handle}>r_{cork}$, then you can with less force at the handle generate enough force at the cork perifery: $$\tau=F_{you}r_{handle}\quad\text{ and }\quad \tau=F_{cork}r_{cork}\quad\Leftrightarrow\\ F_{you}r_{handle}=F_{cork}r_{cork}\quad\Leftrightarrow\quad F_{you}=F_{cork}\frac{r_{cork}}{r_{handle}}.$$ Since it is now this new force, $F_{cork}$, that must overcome static friction, $F_{cork}>f_s$, and not your own pulling force, and since the twisting force, $F_{you}$, you apply is smaller than, $F_{you}<F_{cork}$, then it is much easier to make the cork rotate and thereby overcome static friction, and then apply a subsequent pulling force that easier overcomes the smaller kinetic friction.

enter image description here

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – ACuriousMind
    Jul 30 at 11:23
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As Steeven said, kinetic friction is a smaller force than static friction. Once the cork is moving in any direction, it is easier to move in the direction you want.

As Anna V said, bonds may be broken that make it easier to move a second time after it has moved once.

So why is rotation easier than longitudinal movement? Think of a screwdriver. A large diameter handle allows you to exert a large torque on the screw. That is, small forces a large distance from the axis apply large forces a small distance away. So the large handle on the corkscrew helps.

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Let me take a shot:

The cork has made chemical bonds with the glass. These are the same for all dS of the cork surface: the difference is that in rotating the cork because of the circular motion, a small d(theta) brings the surface unstuck, the resistive forces will not add( different directions). For the axial direction the surface is continuous and the forces needed are additive. Once it becomes unstuck then axial force is effective, because it takes time for the bonds to form, between cork and bottle.

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    $\begingroup$ Good point. On a purely "materials science" point of view, there is slightly more risk of shattering the cork if only longitudinal force is used to initiate motion. This is a small risk, and there are corkscrews with large levers that do only apply longitudinal force. $\endgroup$ Jul 26 at 13:37
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    $\begingroup$ An interesting hypothesis. Proposed experiment: get multiple bottles, and twist the cork of all but 1, but don’t move laterally at all. Wait various amounts of time and then see if additional force is required as time goes on. $\endgroup$
    – Tim
    Jul 26 at 21:43
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    $\begingroup$ Any chemical bonds are solely secondary chemical bonds such as dipolar or hydrogen. An equally likely conjecture is that, over time, the cork conforms more tightly to (fills into) micro-structural deformities on the glass surface, increasing the mechanical component of friction. The frictional force at any direct point of contact between the cork and the glass wall is the same in all directions. The net external force that must be applied to move the cork against the frictional force is therefore the same regardless of the direction to move the cork. $\endgroup$ Jul 27 at 12:53
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    $\begingroup$ @Tim - interesting. I tried with several bottles, and when the hangover goes, I'll try to remember what the experiment was about... $\endgroup$
    – Tim
    Jul 27 at 13:13
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    $\begingroup$ A $d\theta$ rotation still has to translate to a $ds$ arc length motion ($ds = R d\theta$). In the end, you are still trying to move through a $dz$ linear length. The motion is additive across the entire cork surface, so whether you initially do rotational or linear translation to unstick makes no difference. What makes rotational easier than linear translation is the magnifier obtained by the torque lever arm. $\endgroup$ Jul 27 at 13:21
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Its all angles and body geometry.

So obviously the cork can be pulled straight out. That's what corkscrews do. So we know the cork can handle the stress of being pulled out, or rotated. It must be in the hands.

If you pull the cork out, think of what muscles you are using. You have to use your shoulder and upper arm muscles. Look at the leverage you have. Not much. Contrast that with rotating the cork. If you rotate the cork, its a rotation about a very small axis, and your body is designed to be able to clamp down and direct all of its muscular force into generating that torque. Just estimating, a cork is about 12mm in radius, and the human arm is around 600mm, so you have around a 60x mechanical advantage when turning.

Once its turning, this gets to Steeven's answer. The turning means we no longer deal with the static friction of the cork against the glass. We only deal with its kinetic friction. This is typically much lower, so now it is much easier to use those big muscles axially.

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  • $\begingroup$ Kinetic friction is only relevant if you pull WHILE still turning, but that isn't necessary - twisting and THEN pulling also improves your chances. Seems obvious to me that initially the cork has strong bonds with the bottle - twisting breaks these, meaning that a straight pull then only has to overcome 'pure' friction. Similar effect with removing a seized bolt - the first degree of turn breaks long-term bonds, further turns are MUCH easier, despite a negligible change in total friction. $\endgroup$ Jul 28 at 16:04
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    $\begingroup$ @MikeBrockington I have opened hundreds of Prosecco bottles: first you apply torque and when the cork moves, only when the cork moves, you can apply just the slightest longitudinal force and because there is only kinetic friction, already won by the torque, the cork moves in the longitudinal direction. Next, the seasoned Prosecco opener feels when the friction on the reduced contact area is just sufficient to equilibrate the CO₂ pressure, so they stop twisting and pulling and start pushing. BANG! This is how a Prosecco bottle must be opened. $\endgroup$
    – gboffi
    Jul 29 at 14:29
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    $\begingroup$ Many corks are not strong enough to be pulled straight out from the top. Corkscrews distribute the force through the full length of the cork, so that a straight pull will not rip the cork into pieces. $\endgroup$
    – jwdonahue
    Jul 29 at 20:13
  • $\begingroup$ @gboffi You are again saying that there is "only kinetic friction" on a static cork. If you twist a cork AND THEN PULL then static friction is inevitable, but that action DOES reduce the force required - I too have opened enough bottles to know this anecdotally. $\endgroup$ Jul 30 at 9:08
  • $\begingroup$ @MikeBrockington Don't stop twisting when the cork starts moving. Keep the cork rotating about the longitudinal axis (i.e., in a condition of kinetic friction) while you mildly pull. I understand that this contemporaneity of actions was not clearly stated in my previous comment, but… Did you stop twisting before pulling? $\endgroup$
    – gboffi
    Jul 30 at 9:27
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tl;dr Twisting the inside of a cork can compress it, making it easier to remove. The same method is used with foam earplugs.


Twisting makes the cork smaller.

Materials tend to get compressed when twisted internally.

Examples:

  1. Foam earplugs.
    Foam earplugs are basically the same thing as corks, in that they're stoppers made of somewhat-compressible material. To insert such earplugs, people roll them into a tighter configuration, which compresses them. Then they can go into an ear.

  2. Fork in spaghetti.
    To eat spaghetti, folks will often insert a fork and twist the fork around. This causes the spaghetti to tighten around the fork, making it easier to pick up.

  3. Drying fabric.
    Say a towel or other fabric is wet. Then, someone might twist the material tight to compress it, forcing out the water inside.

  4. Threading a needle.
    Needles have little holes on them for thread to go through. But frayed thread can be a bit too big to get in; so, someone might twist the thread to compact it, making it much easier to thread a needle.

So, if someone twists a cork, it may help compress it.

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  • $\begingroup$ That would be true if the poison's ratio was positive, but interestingly enough a cork has the weird property of $\nu = 0$ wiki. $\endgroup$
    – JAlex
    Jul 30 at 16:28
  • $\begingroup$ @JAlex: Poisson's ratio would be for perpendicular force, which would be like compressive/expansive force due to pushing/pulling. Internal twisting would be more like compressive radial pressure, where the cork would compress unless it had a non-positive compressibility. $\endgroup$
    – Nat
    Jul 30 at 18:08
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Interesting question. Here is how I would explain it -

It is important to note that it is NOT a necessity to pull while twisting the cork, if you have appropriate openers that use leverage, then its probably the best and easiest way to go about it.

So this question can only be answered in the context of NOT having appropriate tools. To be precise in the answer let select such a tool, the most common way is to use a flat small knife and pierce into the cork then pull it slowly while rotating it.

Breaking this problem in all the working components, we have,

  1. The knife that's used as an opener
  2. the cork
  3. Human muscle, which provides the essential force
  4. The bottle neck in contact with the cork

So WHY DOES ROTATING WHILE PULLING MAKES LIFE EASIER?

  1. In the context of the tool you use -

As already mentioned, unlike bottle caps (that has thread on it) rotating a cork to open it is not a necessity. But whether you want to rotate it highly depends on the tool you use. In our case we are using a knife. Now you can pretty well see what's gonna happen if I just pull it. The knife will come out just as the way it went in, meaning there is no hook or grip on the knife, other than friction between knife surface and cork, to hold the cork while its being pulled out. So the only way is to slowly pull the system using a rotatory motion.

What does rotation generate? Among other things that is discussed below, it generates a higher pressure between the knife surface and the cork (you are essentially pressing the knife surface hard against the cork), hence INCREASING THE FRICTION. Now that we have increased friction, we can use more force than that would have been possible if we weren't rotating.

  1. The cork -

The cork is freshly cut (meaning its very likely being used for the first time), hence the surface is usually very rough. In fact this is essentially the reason why you cant use a cork multiple times without leakage. When you rotate, you are essentially doing the job of what a stone grinder would do. You are grinding and polishing the surface of the cork in contact with the bottle. Now, its a known fact that, smooth surfaces move around easily as compared to rough surfaces (this is essentially the reason why vehicle tyres have various pattern on it and not bald).

  1. The Physiological side of it-

You are applying force using human hands, and any discussion regarding the same would be incomplete without considering them. It can easily be verified that rotating the wrist is very different from pulling your arm closer to you, meaning the muscles involved are different in both the context (Check it out yourself by trying to rotate the wrist and then pulling something all the while feeling which muscles are tense)
Since we are using different muscles in combination, it is natural to feel more easier to rotate and pull vs just pull.

  1. Bottle neck -

Notice the the bottle neck is almost always more wider on the outside than the inside (and so corks also have the same shape). This essentially helps in redirecting the normal force due to the cork in slightly outward direction (reaction is perpendicular to the surface and the surface here is included). Hence in a vague sense, you are essentially using the rotational motion to generate an upward force, contrary to the usual situation where everything (torque, motion, etc) in a rotational motion is perpendicular to the axial motion.


Point 4 can also be applied to say that rotational motion essentially changes static friction (due to chemical bonding as explained in another answer by @annav) to kinetic friction, which remains kinetic in the axial direction despite no motion along the axis (elaborating the answer by @steeven).

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The initial twisting step is just ergonomically easier than the initial pull it would be require to get the cork moving. For this you have to overcome the static friction between cork and glass. Also there may be van der Waals forces that need to be broken before the cork starts moving. Once the cork moves the friction is lower and it can be pulled. Note that mechanical devices don not bother to twist, they just pull.

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Why does twisting a cork make it easier to remove from a bottle?

When you apply an axial force and, possibly, a torque to the cork (and, opposite, to the bottle), you generate a state of tangential stress between the cork and the bottle neck.

With $A$ being the area of contact, $N$ the axial force and $W$ the torque, the longitudinal component is $\tau_{rz}=N/A$ and the tangential component is $\tau_{r\phi}=W/(rA)$, $r$ being the inner radius of the neck.

Because the cork begins to move when the resultant of the tangential stress between the cork and the bottle's neck

$$ \tau=\sqrt{\tau_{rz}^2+\tau_{r\phi}^2} \tag{1} $$

exceeds the friction bond, you can recognize that applying a torque (twisting the cork) reduces the axial force $N$ that you need to exert.

After establishing that applying a torque reduces the $N$ needed to start moving the cork, when the cork starts moving it's all down hill because the friction coefficient is different (static vs dynamic friction) and also the surface of contact is continuously reduced.

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