Braking while Accelerating

[Xyril]

Um, it totally does? That's basic thermodynamics--the rate of heat transfer from hot to cold is dependent on the temperature differential between the two.

Umm, I think that would more accurately be described as cooling velocity. Unless you want to go really deep into the weeds on temperature being essentially kinetic energy, temperature is a state and not a rate.

There has to be something else involved in the hot water/cold water freezing thing than regular heat transfer, though, because if that's all it was the hot bowl would cool faster than the cool bowl, but wouldn't overtake it (it cools faster, but has further to cool, so the two things should balance out).

If you read the link, they explicitly articulate four mechanisms that might be powerful enough to give you a different result than what you'd get purely by considering Newton's law of cooling. I'm not entirely sure I buy the explanations--in particular, I see no reason why the convection currents they cite wouldn't still occur in the cooler water--but it's there and it's clear.

[Elkad]

Don't do it too much or you'll mangle your brake pads.

Brake pads are cheap. Unless you drive a supercar with carbon-ceramic exotics, it's $100-ish to throw a new set of "sporty" pads on all 4 wheels. (Plus an hour of your time of course)
The parts-chain ones are under $20/axle, if you don't mind terrible pedal feel, heat fade, bad fits that bind calipers, and uneven wear that reduces performance and warps rotors.

[Peelee]

Brake pads are cheap....it's $100-ish.

Look, I get that its cheap for car parts, but I still bridle at that being called cheap.

[factotum]

Look, I get that its cheap for car parts, but I still bridle at that being called cheap.

How much did you spend on your car? I bet, compared to that, $100 is chicken feed.

[Peelee]

How much did you spend on your car? I bet, compared to that, $100 is chicken feed.

No argument on that, but any triple-digit sum hitting all of a sudden, while not putting me out of house and home, is certainly something to work into the budget, which is my threshold for "not cheap."

[Lord Torath]

If you read the link, they explicitly articulate four mechanisms that might be powerful enough to give you a different result than what you'd get purely by considering Newton's law of cooling. I'm not entirely sure I buy the explanations--in particular, I see no reason why the convection currents they cite wouldn't still occur in the cooler water--but it's there and it's clear.

Convection currents will form in the cooler water, but they will be weaker than those that form in the warmer water, and will not last as long.

[Xyril]

Convection currents will form in the cooler water, but they will be weaker than those that form in the warmer water, and will not last as long.

Yes, but where I have my doubts is in whether those currents will remain stronger even after the average temperature is the same as the starting point of the cooler water.

Say for example we have 40 degree water and 60 degree water. I can see convection currents causing the water to free faster if the initial energy in the hotter water is able to support such strong convection currents that, by the time the average temperature of the system is 40 degrees, the system is not uniform in temperature and is constantly cycling hotter (> 40) water to the surface, such that the release of heat is much faster than that of the uniformly 40 degree water. Where I am skeptical is the idea that this could happen (at least, with substantial enough temperature variation to make a difference) in such a small system. When such small masses and distances are involved, I don't think you'd have such drastic temperature differences for long before conduction and mixing started evening things out, so my gut feeling is that by the time the 60 degree water hit the 40 degree average temperature, there might be a bit more kinetic energy and a bit less uniformity than the water that began as a still, uniform 40 degrees, but not enough for the hot water to make up for lost time in the race to freezing.

[Xyril]

No argument on that, but any triple-digit sum hitting all of a sudden, while not putting me out of house and home, is certainly something to work into the budget, which is my threshold for "not cheap."

Not to mention the fact that driving in a way that helps you to reduce how often you need to pay that expense overlaps a lot with driving in a way that's safer.

[georgie_leech]

Oddly enough the former actually has no bearing on the latter. It can become very problematic when you try and recncile those two things however.

Feynman was a jerk? I liked his book.

Feynman was a brilliant mind, but as part of that same curiosity that drove him to push the limits of science, he tended to push other limits as well. The guy tended to push boundaries, including those of other people. So in a sense they do actually have some bearing on each other; the bits of him that make him a bit of a wise ass and a character in one context can be woefully creepy in another.

[halfeye]

Yes, but where I have my doubts is in whether those currents will remain stronger even after the average temperature is the same as the starting point of the cooler water.

Say for example we have 40 degree water and 60 degree water. I can see convection currents causing the water to free faster if the initial energy in the hotter water is able to support such strong convection currents that, by the time the average temperature of the system is 40 degrees, the system is not uniform in temperature and is constantly cycling hotter (> 40) water to the surface, such that the release of heat is much faster than that of the uniformly 40 degree water. Where I am skeptical is the idea that this could happen (at least, with substantial enough temperature variation to make a difference) in such a small system. When such small masses and distances are involved, I don't think you'd have such drastic temperature differences for long before conduction and mixing started evening things out, so my gut feeling is that by the time the 60 degree water hit the 40 degree average temperature, there might be a bit more kinetic energy and a bit less uniformity than the water that began as a still, uniform 40 degrees, but not enough for the hot water to make up for lost time in the race to freezing.

Where it goes funny is at 4 degrees C. That's the point at which water achieves peak density, hotter than that floats, cooler than that floats. Something goes quicker in the change-over with the water that starts hotter. How it stays at that temperature while losing energy is interesting.

[Rockphed]

Where it goes funny is at 4 degrees C. That's the point at which water achieves peak density, hotter than that floats, cooler than that floats. Something goes quicker in the change-over with the water that starts hotter. How it stays at that temperature while losing energy is interesting.

The interesting thing I found in the linked article was that whether or not this effect appears is based on the starting temperatures of the water (they have to be far enough apart, but not too far apart), and the experimental set-up. I suspect that actually explaining why warmer water can freeze before colder water would involve a significant amount of fluid mechanics, thermodynamics, and crystal theory.

Hmmm, here is an experiment that might be interesting: If we have a stream of water drops dripping in a cold environment, how does water temperature relate to distance the drops can fall before they are solid (or at least solid enough to remain intact) when they hit the bottom? Presumably there is also a relationship between air temperature and falling distance as well.

[snowblizz]

Feynman was a brilliant mind, but as part of that same curiosity that drove him to push the limits of science, he tended to push other limits as well. The guy tended to push boundaries, including those of other people. So in a sense they do actually have some bearing on each other; the bits of him that make him a bit of a wise ass and a character in one context can be woefully creepy in another.

I'm speaking of the consumer end so to speak. I can like a work and dislike the creator. I can like a creator and dislike the work. However in practice those tend to influence each other. Very often people liking a work, means they feel like they have to embrace the creator as well.

Like Belkar in OOTS. Many many people work hard to making him look better because they enjoy the character. However he is Evil. Therefore I am maybe Evil as well? The solution is to twist reality so Belkar isn't Evil at all.

[Elkad]

Look, I get that its cheap for car parts, but I still bridle at that being called cheap.

Sure. If you are scrounging for gas money, you run the cheaper ones. I have (and still do on one of my vehicles). Which is why the $16.99/axle pads sell like mad at the chain stores. The price of lunch for two gets you 30,000 miles on the front, or double that on the back, and they'll get you to and from work just fine.

I consider flinging my car down the twisties part of my entertainment budget, so I buy better parts.

[jqavins]

About the original topic, and sorry if this has been stated already in among the posts about freezing hot water, there seems to be a factor that's been left out. While the equations of motion do, indeed say that immediately the preceding acceleration is irrelevant, what we're talking about here is not a mass on a blackboard in PHYS 101, it's a car. Hard acceleration means a low gear and engine rev'ed high. When the foot comes off the gas, the engine speed does not drop immediately The engine's inertia is fighting against the braking effort, and immediately after hard acceleration, the low gear means that the engine will still be pushing on the drive shaft harder that it would immediately after sedate cruising.

A small difference to be sure, but a difference all the same.

(I'm a physicist at heart, and an engineer by training.)

[factotum]

When the foot comes off the gas, the engine speed does not drop immediately The engine's inertia is fighting against the braking effort, and immediately after hard acceleration, the low gear means that the engine will still be pushing on the drive shaft harder that it would immediately after sedate cruising.

Myself and Lord Entralis already discussed that on page 1.