Braking while Accelerating

[Caesar]

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. 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).

That doesn't explain the effect. The formula you are citing says that the instantaneous rate of heat transfer will be faster while the hot object is hot. Once the hot object gets down to the low temperature, which takes time, the rate of heat transfer will of course be the same as for an already equally low temperature object (here assuming the fridge is even lower temperature).

In other words, if you put water at 90° C into the freezer, it will very quickly lose temperature, but once it gets to a lower temperature, lets say 5° C (arbitrary), the instantaneous rate of temperature loss will now be lower, and equal to another 5° C object put into the same freezer. So why, if it takes time to go from 90° to 5°, and an equal amount of time to go from 5° to 0° for both objects, does the hot object objectively freeze faster? Clearly the simple formula does not take into account enough factors to accurately explain the phenomenon.

[Caesar]

If I am accelerating on my car and I suddenly have to stop, how does acceleration affect my braking? Do I stop in the same time as usual for the speed I was travelling at? Or does it take more time because I was accelerating?

If you are actively accelerating (lets say you dont take your foot off the gas when you stomp the brakes), then it will take more energy and thus a longer time to stop when you hit the brakes. Note that gravity can accelerate your car down an incline, even while in gear.

If you take your foot off the gas and the car isn't being accelerated by an external source, then the stopping time is unchanged.

[Knaight]

U. Cal seems to back it up. Of course, that's not one of the super famous U. Cals, but i assume the physics department knows what it's taking about.

The physics department really needs to talk to the chemistry department on this one - there are supporting calculations for dissolved gas having an effect, starting with the entire phenomenon of freezing point depression being really well documented. The same thing applies to mixture enthalpies. That said, aside from that quibble it's a solid article.

[factotum]

That doesn't explain the effect.

Of course it doesn't. You'd almost think I would have added something to my post to that effect (maybe starting with "There has to be something else involved in the hot water/cold water freezing thing"), but obviously I didn't do that, so your post correcting me was entirely justified.

[halfeye]

Of course it doesn't. You'd almost think I would have added something to my post to that effect (maybe starting with "There has to be something else involved in the hot water/cold water freezing thing"), but obviously I didn't do that, so your post correcting me was entirely justified.

If someone looks at the graph, they both do more or less as expected down to 4 degrees C, then they both stay there for longer than it took them to get down there, but the originally cooler water stays at 4 degrees C for about twice as long as the initially hot water does, and that's where the initially hot water overtakes it. Water does peculiar things at 4 degrees C, and this appears to be another of them.

[Peelee]

The physics department really needs to talk to the chemistry department on this one

Forget XKCD, there's an SMBC for everything.

Spoiler: I'm sure they will any day now.

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[LordEntrails]

If you take your foot off the gas and the car isn't being accelerated by an external source, then the stopping time is unchanged.

As discussed previously in this thread, that's not true. Because their are internal sources of momentum and other small factors.

[Jimorian]

Speaking of braking while accelerating allows me to post one of my favorite videos. A Boeing 777 undergoing "Rejected Takeoff Test" where the plane is loaded to maximum takeoff weight, sent down the runway until it's just short of takeoff speed, then slams on the brakes WITH THE ENGINES AT FULL THRUST! Yes, the brakes can outperform 2 engines with 115,000 lbs of thrust each. Yes, they get very very hot.

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[Peelee]

Speaking of braking while accelerating allows me to post one of my favorite videos. A Boeing 777 undergoing "Rejected Takeoff Test" where the plane is loaded to maximum takeoff weight, sent down the runway until it's just short of takeoff speed, then slams on the brakes WITH THE ENGINES AT FULL THRUST! Yes, the brakes can outperform 2 engines with 115,000 lbs of thrust each. Yes, they get very very hot.

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OK, that's just badass. I can't imagine a situation where that would really be an issue to have, but it's awesome that we can test for it and achieve the desired result just in case.

[Rockphed]

Speaking of braking while accelerating allows me to post one of my favorite videos. A Boeing 777 undergoing "Rejected Takeoff Test" where the plane is loaded to maximum takeoff weight, sent down the runway until it's just short of takeoff speed, then slams on the brakes WITH THE ENGINES AT FULL THRUST! Yes, the brakes can outperform 2 engines with 115,000 lbs of thrust each. Yes, they get very very hot.

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Ah, the actual answer to the old "jetplane on a treadmill" question. Thank you for the video.

[Peelee]

Ah, the actual answer to the old "jetplane on a treadmill" question.

Well, it depends on how fast the treadmill is moving relative to the ground.

[Lord Torath]

If someone looks at the graph, they both do more or less as expected down to 4 degrees C, then they both stay there for longer than it took them to get down there, but the originally cooler water stays at 4 degrees C for about twice as long as the initially hot water does, and that's where the initially hot water overtakes it. Water does peculiar things at 4 degrees C, and this appears to be another of them.

That's about the point where the density starts doing funny things. Until you hit 4^oC, density is increasing as temperature is decreasing: cold water sinks, hot water rises, you know the drill. Below 4^oC, that reverses, and the density drops as you approach freezing. Now the colder water wants to float on the less-cold water, which is why lakes ponds, streams, and the ocean freeze from the top down, rather than the bottom up.

[LordEntrails]

That's about the point where the density starts doing funny things. Until you hit 4^oC, density is increasing as temperature is decreasing: cold water sinks, hot water rises, you know the drill. Below 4^oC, that reverses, and the density drops as you approach freezing. Now the colder water wants to float on the less-cold water, which is why lakes ponds, streams, and the ocean freeze from the top down, rather than the bottom up.

And why ice skating works, at the right temp, adding pressure to ice causes it to melt, therefore providing a lubricant to skate on.

[LordEntrails]

Speaking of braking while accelerating allows me to post one of my favorite videos. A Boeing 777 undergoing "Rejected Takeoff Test" where the plane is loaded to maximum takeoff weight, sent down the runway until it's just short of takeoff speed, then slams on the brakes WITH THE ENGINES AT FULL THRUST! Yes, the brakes can outperform 2 engines with 115,000 lbs of thrust each. Yes, they get very very hot.

Thanks for that. An impressive feat of engineering.

[Iruka]

And why ice skating works, at the right temp, adding pressure to ice causes it to melt, therefore providing a lubricant to skate on.

The jury is still out on that.

BTW, you can 'collect' posts to quote with the '+'-button on the lower right, so you don't have do double post or mess around with copy/paste.

[enderlord99]

I still don't understand why all scientists hate other types of scientists, which they claim aren't "real" scientists.

[Tvtyrant]

I still don't understand why all scientists hate other types of scientists, which they claim aren't "real" scientists.

I'm reasonable certain it is a joke. Just like Anthropologists.

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Joking of course.

[Peelee]

Yeah, it's more of a friendly rivalry. As always, there's a delightful SMBC explaining how silly an actual rivalry would be.

[snowblizz]

Yeah, it's more of a friendly rivalry. As always, there's a delightful SMBC explaining how silly an actual rivalry would be.

Actually. No, no it's not. There may be a lot of joking and near jokes. But let's be clear there is absolutely rivalry and even hatred within fields and between them in science. It's not the most common situation, but it's not noteworthy uncommon either.

I still don't understand why all scientists hate other types of scientists, which they claim aren't "real" scientists.

It's not quite that bad. Besides, scentist can't hate all other scientist for not being real scientists. They hate all those people claiming to be scientists when they aren't real scientists like they are.

It's absolutely real that natural science often looks down on other sciences. I know several fields who modify their behaviour to try and emulate "hard" science for that reason. The character Sheldon in Big Bang Theory is a humorously overblown stereotype, but the kernel of his being rings of truth.

The main "problem" is that scientists are human, and not a lot like the rational philosopher type they'd like you to think. Humans have human issues. (And unlike scientists in the humanities the natural scientists can't see theirs ;P)

The second issue can be spellt "competition for resources". Any funding someone else gets is less money for you.

Third, clashing theories. This is one of the worse ones for conflict. Two subjects looking at the same thing from slightly or wildly different angles.

I read a piece in the Atlantic not long ago aptly named the Nastiest Feud in Science. Which is an example of science at it's worst. Hitting points one and two.

Sometimes if you combine all three you can get a super-hurricane of rancor.

[Knaight]

Actually. No, no it's not. There may be a lot of joking and near jokes. But let's be clear there is absolutely rivalry and even hatred within fields and between them in science. It's not the most common situation, but it's not noteworthy uncommon either.

For instance, there's the use of the dismissive term "stamp collecting" in dead serious ways, mostly by physicists and towards every other field.

[snowblizz]

For instance, there's the use of the dismissive term "stamp collecting" in dead serious ways, mostly by physicists and towards every other field.

Exact words used by one side about the palentologists in the article I linked from the Atlantic! Have you read it too?

[Knaight]

Exact words used by one side about the palentologists in the article I linked from the Atlantic! Have you read it too?

I have, but I'm familiar with that term from much earlier. For instance, Feynman famously referred to the entire discipline of biology as stamp collecting.

[Rockphed]

I have, but I'm familiar with that term from much earlier. For instance, Feynman famously referred to the entire discipline of biology as stamp collecting.

And the greatest biologists have slain one of the 4 horsemen of the apocalypse, while the heroes of physics gathered in the desert to create a new one.

Personally, I try to maintain a healthy respect for people whose disciplines I am not well versed in. Then again, I am an engineer, not a true scientist. Engineers practically revel in their ability to get things done with only a half formed idea of how things work.

[Peelee]

Actually. No, no it's not. There may be a lot of joking and near jokes. But let's be clear there is absolutely rivalry and even hatred within fields and between them in science. It's not the most common situation, but it's not noteworthy uncommon either.

Well, sure, every discipline will have egotistical nutjobs who think they're gods gift to the one true science, but that doesn't make them the norm. Take Feynman, who really seemed like a nice guy.

[Rockphed]

Well, sure, every discipline will have egotistical nutjobs who think they're gods gift to the one true science, but that doesn't make them the norm. Take Feynman, who really seemed like a nice guy.

I think it is something in physicists, personally. Feynman was a physicist, the guy who turned the "what killed the dinosaurs" debate toxic was a physicist. Based on 2 data points, all famous physicists are jerks.

[LordEntrails]

I think it is something in physicists, personally. Feynman was a physicist, the guy who turned the "what killed the dinosaurs" debate toxic was a physicist. Based on 2 data points, all famous physicists are jerks.

Feynman was a jerk? I liked his book.

[factotum]

Feynman was a jerk? I liked his book.

See the link in Peelee's post above--it certainly implies Feynman had some odd ideas about what is acceptable behaviour toward women, at the very least.

[Rockphed]

See the link in Peelee's post above--it certainly implies Feynman had some odd ideas about what is acceptable behaviour toward women, at the very least.

I haven't even followed the link, I just read the title out of the link. I'm sure I could hunt down other physicists who were jerks in a very public way, but that doesn't seem helpful.

[snowblizz]

Feynman was a jerk? I liked his book.

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.

[wumpus]

And the greatest biologists have slain one of the 4 horsemen of the apocalypse, while the heroes of physics gathered in the desert to create a new one.

Personally, I try to maintain a healthy respect for people whose disciplines I am not well versed in. Then again, I am an engineer, not a true scientist. Engineers practically revel in their ability to get things done with only a half formed idea of how things work.

I'm not sure where in the physics pantheon Feymann sits, but it has to be roughly as high as where James Watson would be on the discovery of the structure of DNA. Feymann might be judged as a jerk as time rolled past and acceptable standards changed (granted, Feymann didn't care too much about acceptable standards anyway: thus the title "surely you are joking Mr. Feymann"), but Watson managed to get himself shunned in his own time.