Wikipedia:Reference desk/Archives/Science/2020 January 30

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January 30

Car rolling down slope

If I park my car on a slope and don't apply the handbrake, if the gradient is steep enough, it will eventually start rolling because (as I understand it, but correct me, I'm no scientist) the forces exerted by gravity overcome friction/inertia.

However, if I park on a slope that's fairly gentle, there seems to be a delay before the rolling kicks in.

What is it that is happening during the interlude that turns lack of movement into movement? Why doesn't it start rolling immediately? Presumably gravity and friction are constant during that time. Let's exclude outside factors like wind. --Dweller (talk) Become old fashioned! 13:58, 30 January 2020 (UTC)

If we exclude outside factors like wind, then there is no movement. Wind is often the culprit in such situations, as if there is a stable equilibrium between static friction and gravity, there would need to be some outside force (wind, a cat rubbing up against it, a bird landing on it, etc.) to upset that equilibrium. Be aware that static friction is always greater than moving friction (such as rolling friction or sliding friction), so there only needs to be just enough force to get it moving, from that point gravity is sufficient to do the rest. Simply put, if a stationary car starts moving, it is absolute proof that there IS some "outside factor like wind." that got it started. --Jayron32 15:01, 30 January 2020 (UTC)

If the slope is steep enough, a wheeled device without brakes is likely to start rolling immediately. The next question might be, what is the threshold for sufficient steepness for that rolling to start? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:08, 30 January 2020 (UTC)

It's hypothetically calculable, but there's a LOT of variables to consider, including (but not limited to) size and type of tires, mass and shape of car, type of pavement, is it wet or dry, if it is windy or not, how much friction there is in the car axles, etc. etc. --Jayron32 15:28, 30 January 2020 (UTC)

So various friction components, assuming stability otherwise, could be enough to keep it from going anywhere when there's only a shallow slope. Analogous to a shopping cart. If the pavement is sufficiently close to being level, the cart will stay put during unloading. If it's steep enough (even a few degrees), it will start rolling unless you constrain it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:40, 30 January 2020 (UTC)

That's true, but it is true of any object, not just a car. If you were standing on the ground, and the slope suddenly started to increase, you could maintain your position until the force of gravity overcame the friction between your shoes and the ground, and which point you'd start sliding down the hill. --Jayron32 15:45, 30 January 2020 (UTC)

For sure. The difference is that you can adjust your posture to try to keep from sliding. An inanimate object can't do that. The OP's original statement that "the forces exerted by gravity overcome friction/inertia" is not blanketly accurate. It's only true when the slope is sufficient for the force of gravity to take over. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:49, 30 January 2020 (UTC)

Not really, You can adjust your posture to keep from falling over, but no amount of posture adjustment will have a significant impact on the friction between your shoes and the slope, and once you start sliding, you ain't stopping. --Jayron32 15:51, 30 January 2020 (UTC)

That would again depend on the steepness angle. Although if you insist on trying to stay on your feet in that situation, as opposed to falling backward and changing your situation, it's probably as you said. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:59, 30 January 2020 (UTC)

The tires don't immediately deform if the tendency to roll is slight. It may take a moment for tire deformation to take place, and once tire tire deformation has taken place, inertia may now become a force favoring motion whereas inertia when the car was first stopped would tend to favor a body remaining at rest. Deformation of the tires is an important prerequisite of a rolling automobile. When you initially brought the car to a stop you used the normal foot brakes to bring the car to a stop. At that point the deformed tires are essentially a wheel chock, if the forces favoring a rolling motion are slight. "[A] rubber tire will have higher rolling resistance on a paved road than a steel railroad wheel on a steel rail." Bus stop (talk) 16:40, 30 January 2020 (UTC)

Let's not fall into the trap of assuming that a modern manufactured road-legal automobile is made of four simple wheels that are rigidly connected to simple axles... these machines are complicated.

Among the factors to consider, we have a suspension, a complex axle and/or independent suspension, the differential, ... not to mention the drive coupling stuff like the clutch and the transmission ... and we have bearings and automotive-grade lubricants... and nowadays there are active control systems ... if we were to try to model the torque or the transient response of the wheels, it would not be a simple curve like the ones you see in our articles on friction.

Before you dive into any analysis, it might be good to start with an introduction to modern automobile design. Here's a book you might find at your local library: Automotive Technology: A Systems Approach.

Nimur (talk) 16:43, 30 January 2020 (UTC)

• You're not on a flat slope, you're on a tiny mountain range. It's lumps and bumps.

If you roll an infinitesimal amount on a smooth slope, you're going to the bottom (however slowly).

However if you wobble a bit on a rough slope (and with deformable tyres) you might have the momentum to roll a bit further (i.e. to potentially go all the way), except that this might not be enough to get past the next minor lump of gravel, or tyre tread block. So you stop. Wobble a bit harder (even just bouncing vertically up and down, as many kids left in cars have found) can be enough to get over that step. If you've the momentum to get over the first representative hump, and there's a slope to gain a little bit more, then you've probably got enough to get over the next. Then the next (unless it's appreciably bigger) and on it goes. Andy Dingley (talk) 16:51, 30 January 2020 (UTC)

The questioner is no doubt aware of the greater complexities involved. But they only mention parking the car, which means bringing it to a stop, and not applying the handbrake. They are asking why, if it eventually rolls, does it sometimes not start rolling immediately. Yes, wind may be a contributory factor, as Jayron32 says, but I find that Rolling resistance gets more to the heart of the matter on a theoretic level. An inflated tire is deformed when the car is at rest. It doesn't have the minimized rolling resistance of "a steel railroad wheel on a steel rail." Bus stop (talk) 16:58, 30 January 2020 (UTC)

• "Parking doesn't mean the handbrake" is only in North America (and influences). It would be a specific fail on the UK driving test to do that, even for relatively short stops, much shorter than parking. Andy Dingley (talk) 17:03, 30 January 2020 (UTC)

But nobody said "Parking doesn't mean the handbrake". To bring the car to a stop they used some sort of brake. Maybe in some parts of the world they use differently-activated braking systems than in my dysfunctional part of the world. We are known to use antiquated inertia resistors. But the point is that after bringing the car to a stop, no permanent or long-lasting braking system was applied. Therefore this question is essentially about a delayed reaction and what could cause that delayed reaction. If rolling eventually transpired but was not immediate, we need to look for factors that can lie dormant for awhile. Bus stop (talk) 17:19, 30 January 2020 (UTC)

The OP said "If I park my car on a slope and don't apply the handbrake" which seems a clear enough statement that they're referring to a situation where someone parks the car but whether by accident or intention does not apply the handbrake and there is no suggestion this is normal or acceptable. You said "But they only mention parking the car, which means bringing it to a stop, and not applying the handbrake." which is fairly confusingly worded. It can easily be read to suggest you think applying the handbrake is not a normal part of parking the car. But Andy Dingley is correct in a lot of of the world applying the hand brake is a part of parking the car. Not doing so would for example, generally be a fail in a driving test. It doesn't matter if the car is an automatic or manual. Nil Einne (talk) 05:43, 31 January 2020 (UTC)

P.S. Although I stick by my above comments, in NZ at least I think it's rare you'd fail from not applying the hand brake for parking since there's no real opportunity to assess it. When start your test, you get into the car first so the instructor can check the lights etc. So even if you forgot to apply the hand brake, the testing officer will probably simply consider it's possible you've been a bit too enthusiastic and taken off the hand brake already. (And you aren't necessarily the person who parked the car anyway.) Likewise while a reverse parallel park is generally a standard part of the modern restricted test, I don't think it's the norm to penalise you for not applying the hand brake at the end of it. Since it's understood you'll be driving off soon, I think they won't care if you don't apply it, although it's probably a good idea. (Also if the officer asks you if you've completed your parking manoeuvre, I would make sure the hand break is applied and even put the car on park for an automatic or an appropriate gear for a manual.) During the test, I think you're generally asked to park after the first stage of the test while the officer checks if you're eligible to proceed onto the next portion. But again, since you're going to move on I think they won't care if you don't apply the handbrake although again, it's probably a good idea. Especially since I think this is when they'll also tell you if you failed the first portion and you don't want to accidentally move if you're emotional and release the footbrake a bit too much, especially in an auto. At the end of the test, you'll drive back to the centre and park. This is again where you'll be told if you passed or failed. But AFAIK, this is generally done from within the car. I'm not totally sure if these sort of things [1] means they sometimes get out of the car. But even so, by this stage they've basically already passed or failed you. So considering the stress of the situation, I wonder if even if you get out of the car without applying the hand brake before they told you you've passed, they'll actually let you off if the car isn't likely to roll off (e.g. an automatic on park or a manual with wheels facing the curb and gear in the opposite direction of any slope). I mean possibly they won't even notice. During the test, if you don't use the hand brake during a hill start especially in a manual and roll backwards this generally be enough for a fail, but that's a specific problem and often not related to parking per se. I had a quick look for comments and couldn't actually find any suggesting the UK is that different. I.E. it's accepted as standard part of parking the car, but simply not really assessed. Nil Einne (talk) 07:14, 31 January 2020 (UTC)

As a UK driver, let me assure you that failing to apply the handbrake when stopping the car, either at traffic lights, in 'stop-start' traffic conditions, when parking temporarily before and after the "reverse round a corner" exercise, or at the end of the test, will result in or at least contribute towards a failure. Driving itself is inherently stressful (as is taking a test of any kind), especially for a novice, and demonstrating that one can competently and reliably do so without creating potential or actual hazards for other road users (including pedestrians) is the main purpose of the test. Examiners actively look for the evidence of one's competence, and the fact that some experienced drivers know when they can safely deviate from driving-test protocols does not excuse the testee from having to demonstrate all the required skills and knowledge. {The poster formerly known as 87.81.230.195} 90.205.58.107 (talk) 10:35, 31 January 2020 (UTC)

The thing which is stopping the car from accelerating is friction ; if the friction is greater than the net force the car will not move at all. However, the friction is not a constant: it may start high enough to prevent acceleration but then start to drop due to, for example tyre deformation or cooling. As soon as it drops below the critical threshold acceleration can start and the previously stationary car will move. 2A01:E34:EF5E:4640:2C0A:BBF1:8583:7297 (talk) 18:38, 30 January 2020 (UTC)

Inertia is not a force and does not play a role in the car remaining stationary or starting to roll. As indicated by several earlier posters, we have the gravitational force, static friction, and possible outside forces such as wind force. If the car was initially stationary (at rest for more than an infinitesimal period) and then starts rolling, an initial equilibrium most have been disturbed. ("I sense a disturbance in the Force.") Leaving loading/unloading the car and earthquakes and such out of the considerations, a change in the gravitational component is unlikely. Either the static friction has decreased or the net outside force has increased. Lacking such changes, the car will remain at rest. What may happen is that the motion of the car in the first few seconds, although not zero, is so low that it is not perceived.  --Lambiam 20:22, 30 January 2020 (UTC)

There are other factors that could dampen incipient motion but the most obvious one is the necessary deformation of the tires for rolling under the weight of the car. Bus stop (talk) 21:38, 30 January 2020 (UTC)

I agree with Bus stop. The element missing from the earlier analyses was the time-dependent properties of rubber tires. The phenomenon under discussion is not observed in vehicles with steel wheels, operating on steel rails. Dolphin (t) 08:54, 31 January 2020 (UTC)

Wouldn't deformation of the tyres tend to increase friction?  --Lambiam 09:29, 31 January 2020 (UTC)

@Lambiam: Friction is a force and most forces are instantaneous responses to their drivers, and remain of constant magnitude whenever the driver is constant. My reply is alluding to time-dependent properties of some materials, where the force exerted by the material is not constant over time even though the driver is. This is an inadequate explanation, I know, but I don’t see an article that goes directly to the matter. Hysterisis hints at material properties that are time-dependent or non-linear so it might be helpful. Another illustration is to contemplate crunching a sheet of paper into a compact ball and then watch it over the next few minutes. Over time, the compact ball of paper will slowly open up and expand in volume even though no external force is forcing it to do so. As the paper ball opens it may do so asymmetrically so that it suddenly topples and rolls a short distance until it finds a new equilibrium position. I hope this helps. Dolphin (t) 00:46, 1 February 2020 (UTC)

Thanks all, interesting stuff. So it seems that the factor(s) I was looking for was tyres deforming (possibly around lumps and bumps on the road surface). Thank you. And thanks to my invisible friend who emailed. --Dweller (talk) Become old fashioned! 09:15, 31 January 2020 (UTC)

Looking for a chemist who has seen Dark Waters

One day ago, I undid this addition because of WP:V and WP:OR. The user logged out and reverted me and made some minor modifications. As the film is not yet running in Swiss cinemas, I can't verify the doubted claims. Is there any chemist (or natural scientist) around who has seen Dark Waters (2019 film)? --Leyo 21:18, 30 January 2020 (UTC)

I haven't seen the film, but I suspect that what you reverted was mostly right. However I have not checked references to see if this is original research or others commented this about the film. Certainly the statement "perfluorooctanoic acid (PFOA-C8) is ... also known as Teflon" is wrong, but if it is in the film as stated then it is OK to include. However I think it would be important to have a statement in that article that points out the errors in the film. Otherwise readers of our article may believe it is true, and take inappropriate personal actions. The reference [2] points out some errors in the film, but that does not mean that it identified all of them! Graeme Bartlett (talk) 21:58, 30 January 2020 (UTC)

• PFOA certainly isn't Teflon!

It's on the list of "Well of course you wouldn't want any exposure to that!" chemicals, so there's relatively little studied about it, because of course no one would dump it into the environment. But then the US standards are so lax, du Pont went and did so. Andy Dingley (talk) 00:03, 31 January 2020 (UTC)

I am well informed about the case (I've seen The Devil We Know, read the NYT feature etc.) and very well about PFOA. I'm asking about the actual content of the film. It is likely that certain simplifications are made in the film, as the target audience are not chemists. This might have been done with more or less factual errors. BTW: I had mentioned the reference linked by Graeme Bartlett in my edit summary. --Leyo 09:58, 31 January 2020 (UTC)

Have a quick look round the auditorium and see of they're all wearing lab coats and protective glasses? Martinevans123 (talk) 10:20, 31 January 2020 (UTC)

@Graeme Bartlett, Andy Dingley, EllenCT, and Shoy: I found the subtitles file online (just google “srt Dark Waters”) and checked each of the 25 occurrences of the word Teflon:

• Several times when they say “back on Teflon”, “her time at Teflon” etc. they are referring to the Teflon (production) line. No error here.
• After 01:24:24 several very short excerpts of news broadcasts are shown: “Teflon has become such a familiar” – “substance that is also used in clothing and cars. Even in contact lenses.” – “Already found in the blood of most Americans.” If the last one refers to Teflon, it is incorrect, since the compounds found in blood are PFOA and other non-polymeric PFASs. However, what the statement refers to remains unclear.

Therefore the claim made in Dark Waters (2019 film)#Scientific accuracy lacks a basis. --Leyo 22:44, 3 February 2020 (UTC)

Muscle twitches before sleep

A person who is trying to get to sleep may experience an involuntary muscular twitch or spasm of varying intensity before nodding off. Is this a scientifically recognized phenomenon and if so what is the term for it? Freeknowledgecreator (talk) 22:27, 30 January 2020 (UTC)e

Hypnic jerk. Although yes, that does sound like a dance from the 1960s. Martinevans123 (talk) 22:33, 30 January 2020 (UTC)

Also check out Exploding head syndrome, which is fortunately not quite so alarming as it sounds (I speak from experience). {The poster formerly known as 87.81.230.195} 90.205.58.107 (talk) 10:39, 31 January 2020 (UTC)

Almost as nice as a goodnight kiss. Martinevans123 (talk) 10:49, 31 January 2020 (UTC)