Braking while Accelerating

[Vinyadan]

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?

[factotum]

Other than the fact it'll take you a fraction of a second to move your foot from the accelerator to the brake, it won't make the slightest difference. Acceleration doesn't have some sort of memory that says "This thing was accelerating a second ago so it can't be decelerating as hard now!".

[tomandtish]

Factotum is correct. Once the brake is applied, stopping time is the same (unless for some reason you keep your foot on the gas at the same time). The only difference MIGHT be in reaction time.*

*I say Might because most people don't drive with their foot ON the brake, so you'd probably be moving your foot to the brake one way or the other. Unless you were using cruise control or going down a hill your foot was probably on the gas anyway.

[Khedrac]

Totally agree for motion. I was thinking that braking while accelerating is what happens to me occasionally because my shoes are big enough that it is hard to press the brake down without catching the accelerator (which is one reason why I always try to hit the clutch as well when I need to brake hard). Sadly I have driven cars where my foot would catch both the clutch and the accelerator when I start to brake!

The curious one is temperature - take one bowl of hot water and an otherwise identical bowl of cold water outside when the air temperature is below 0 degress C and the hot water will usually freeze first.

[Lord Torath]

The curious one is temperature - take one bowl of hot water and an otherwise identical bowl of cold water outside when the air temperature is below 0 degress C and the hot water will usually freeze first.

Well, maybe. It depends on, among other things, the size and shape of the two bowls, the exact definition of "hot" and "cool", and how far below 0^oC the temperature happens to be.

[Anymage]

I've heard various things about the hot water freezing first. Some people say that it's pure myth, others say that it's true only because boiling water will have some boil off in the process, and it'll take less time for the remainder to freeze. Never personally been bothered to check, but hot water does not have a "cooling acceleration" that will cause it to lose temperature faster than cold water.

[Lvl 2 Expert]

The reason speed works like that is because the amount of motion energy an object has is 1/2mv^2, where m is mass and v is velocity or speed. Current or rather really really recent acceleration is not part of the formula. The amount of energy your brakes can absorb per unit of time depends on a lot of factor in itself, but recent acceleration is not one of them. (Except maybe through some effect of heat, but even for that scenario having braked hard semi-recently is much more important than recent acceleration. Or I guess through some sort of effect where the engine is still putting energy on to the wheels because it has inertia, but you can catch that one by using the clutch in both scenarios.)

[Peelee]

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.

[monomer]

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.

Weird, I was taught that the reason hot water freezes faster was because there is enthalpy required to transform from liquid to gas, and the hot water system has a higher energy content which helps the transformation occur. This isn't really one of the four explanations in the article you posted, though part of it may be touched on in the supercooling effect they mention at the end. You can supercool water in a freezer if you are extremely careful with it and take it out before it has a chance to freeze. You then touch the water, and it will immediately crystallize into ice. Starting with hot water may not have as much supercooling before freezing since the system has the energy required to transform.

[Peelee]

The recipe for supercooling water has always amused me; put water in the freezer. If it freezes, you did it wrong.

[Sermil]

Factotum is correct. Once the brake is applied, stopping time is the same (unless for some reason you keep your foot on the gas at the same time). The only difference MIGHT be in reaction time.*

*I say Might because most people don't drive with their foot ON the brake, so you'd probably be moving your foot to the brake one way or the other. Unless you were using cruise control or going down a hill your foot was probably on the gas anyway.

Unless you were accelerating so hard that you were already skidding. (E.g. your tires were already squealing, like a drag racer at the beginning of a race)

If you are already skidding, you'd either need to (a) try to stop while skidding, which will take much longer than a normal stop (because sliding friction is < static friction) or (b) let go of both the accelerator and brake (and steer in a straight line) long enough for the tires to 'catch' back on the road.

Unlikely on a dry road, but might be an issue on wet or icy ones.

(Disclaimer: I live in Los Angeles. Icy roads are a purely theoretical concept to me.)

[factotum]

hot water does not have a "cooling acceleration" that will cause it to lose temperature faster than cold water.

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).

[Lord Torath]

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).

One of the theories I've heard is that the higher temperature difference in the warmer water sets up a convection current which increases the heat flow to the rest of the fluid. As the liquid cools, inertia keeps the current flowing, letting it keep cooling faster than the water which started out cool. This probably is more effective in containers with insulated sides and bottoms, so that most of the heat transfer occurs at the top surface. This is probably most effective where you have cool air blowing on the containers from one side, leading to un-qual cooling, which sets up the convection current, Of course, if your cold water is at 2^oC, and your hot water is at 75^oC, my money's still going to be on the cold water to freeze first. Also, if your containers are constricted, they might impede the development of the convection current, preventing the warmer liquid from "turbocharging" its cooling rate.

The takeaway here is that if hot water in your ice-cube trays freezes more quickly than cool water, you should use cool water because it's cheaper. First, you have to pour a lot of heat into your water to get it hot, and then you have to pull it all back out and then some to freeze it.

[snowblizz]

The takeaway here is that if hot water in your ice-cube trays freezes more quickly than cool water, you should use cool water because it's cheaper. First, you have to pour a lot of heat into your water to get it hot, and then you have to pull it all back out and then some to freeze it.

Besides hot water tastes funny. Wouldn't want that in icecubes.

[Rockphed]

Besides hot water tastes funny. Wouldn't want that in icecubes.

I'm fairly certain that hot water tastes funny because it has lost gasses and gained minerals. Both are the cause of the cloudy bit in the middle of normal ice cubes. A bit that, I should note, often tastes funny.

[wumpus]

Factotum is correct. Once the brake is applied, stopping time is the same (unless for some reason you keep your foot on the gas at the same time). The only difference MIGHT be in reaction time.*

*I say Might because most people don't drive with their foot ON the brake, so you'd probably be moving your foot to the brake one way or the other. Unless you were using cruise control or going down a hill your foot was probably on the gas anyway.

Once you take into account things like the time it takes to move your foot, you might notice other tiny details that exist in real cars that don't exist in the "spherical cow" world of physics problems. In this case, the physics model is good enough.

One tiny detail is that cars have a suspension. That means that the center of mass changes depending on the acceleration of the car. The car will brake best when all wheels have the same weight on them and (for all but sports and/or race cars with more weight on the rear wheels) this will be while accelerating. This effect will be over in milliseconds, so it is completely irrelevant.

An even smaller issue will be that acceleration will heat up the tires ever so slightly, which may (or may not, depending on the outside temperature) effect braking. Note that tire companies claim that winter tires are needed in cold temperatures regardless of snow thanks to being designed to operate at lower temperatures than summer tires. Jason Fenske, of the Engineering Explained youtube channel explained this effect in great detail and then tested it himself and found that his summer tires stopped even faster in freezing conditions (without snow or ice) than his snow tires (which appeared to surprise him).
https://www.youtube.com/watch?v=TJsV2ORMsms
[note: if you have to deal with temperature colder than this (like Jason does in Idaho) or deal with snow I'd still recommend winter tires. Just don't blindly believe the 40 degree fahrenheit claim of tire manufacturers].

[LordEntrails]

As Wumpus ans Ermil have pointed out, their are minor situation conditionals to be aware of. The other is that if you are in a car accelerating, even after you take your foot off the gas, the vehicle still accelerates for a tiny bit as the engine has rotational momentum, gas cutoff and ignition is not instantaneous, and assuming an automatic transmission, the clutch is not instantly engaged either.

So, yes, it takes longer to come to a stop if you were accelerating to 40mph, than if you were cruising at a steady 40mph, but, I suspect the difference is only a few percent or less. And other factors will impact stopping distance much more significantly.

[factotum]

The other is that if you are in a car accelerating, even after you take your foot off the gas, the vehicle still accelerates for a tiny bit as the engine has rotational momentum, gas cutoff and ignition is not instantaneous, and assuming an automatic transmission, the clutch is not instantly engaged either.

Rotational inertia of the engine I'll grant you, but I can't see that having much effect on anything other than a very large engine. Since modern engines are fuel injected the gas cutoff *is* instantaneous, or near as dammit, because the next cylinder to fire simply won't have as much gas injected; given that the cylinders are firing many times per second, I'm pretty sure any lag there is going to be from that is less than the lag of actually lifting your foot off the accelerator to move it to the brake pedal, which has already been mentioned.

Not sure what you mean by the clutch thing? A conventional automatic transmission doesn't have a clutch, it has a fluid flywheel or (more likely these days) a torque converter. Even if that isn't the case (e.g. you're talking about a double-clutch gearbox or similar) then the gearbox can still change down in a fraction of a second, and unless you're driving very slowly, it would likely be in the same gear whether you're accelerating or not--in fact, it might even be in a lower gear if you were accelerating hard due to kick-down (where the gearbox shifts into a lower ratio to make acceleration better), so you'd actually get more engine braking and would stop slightly faster.

[LordEntrails]

Rotational inertia of the engine I'll grant you, but I can't see that having much effect on anything other than a very large engine. ... slightly faster.

"much of an effect", but an effect. Measurable with scientific equipment, but probably not with a stop watch on a track.

My estimate that it could be as much as a few percent is grossly conservative estimate. Because I'm not willing to run the test or calculate the theoreticals.

My point is, cars are complex systems and their is "stuff" that means absolute or simple replies like "no, their is not an effect or difference" might be accurate within practical limits, but are not actually true statements.

[Bucky]

Speaking of tires, they remember a small amount of acceleration.

There's a moment when you start accelerating where the middle of the wheel rotates faster than the rim of the tire. This twists* the tire a bit before the rim catches up. When you stop accelerating and start breaking, the tire takes a moment to return to its normal shape, then twists the other way.

This effect is small, obviously extending your stopping distance by only a tiny fraction of a tire-diameter. But it's easier to measure than these other effects: draw a straight line down the tire and photograph how much it bends under your acceleration. That's how much it extends your stopping distance by.


*Deformation under a shear force

[LordEntrails]

Speaking of tires, they remember a small amount of acceleration....

I actually got to stand next to (~3 feet) and film a family friend in his drag racer a couple years back. It was all so he could take these measurements (tire deformation) and put into his calculations for his launch computer. Not for stopping distance, but my understanding it was to make sure he maximized traction through the fractions of a second considered "launch".

[Peelee]

I've gotta say, I'm a bit disappointed that a physics question isn't being met with a spherical cow in a vacuum an assumption that the brake pedal is hit at the same time in both instances, with zero delay or overlap with the accerlerator.

[Rockphed]

I've gotta say, I'm a bit disappointed that a physics question isn't being met with a spherical cow in a vacuum an assumption that the brake pedal is hit at the same time in both instances, with zero delay or overlap with the accerlerator.

My understanding is that you can do weird things to a car if you start applying the brake and accelerator at the same time. Not bad, but weird. Like you can force it to skid on certain wheels.

[Peelee]

My understanding is that you can do weird things to a car if you start applying the brake and accelerator at the same time. Not bad, but weird. Like you can force it to skid on certain wheels.

Zero delay or overlap. So not applied at the same time.

Anyway, since when has physics been concerned with practice over theory? That's for the dirty engineers to work out.

[Rockphed]

Zero delay or overlap. So not applied at the same time.

Anyway, since when has physics been concerned with practice over theory? That's for the dirty engineers to work out.

... I happen to be an engineer.

...

I'm going to go away before people further insult my personal hygiene.

[Peelee]

... I happen to be an engineer.

...

I'm going to go away before people further insult my personal hygiene.

Oh, not your personal hygiene, just your desire to solve problems without unrealistically altering the conditions in order to make the math more elegant. It is a silly desire.

[Rockphed]

Oh, not your personal hygiene, just your desire to solve problems without unrealistically altering the conditions in order to make the math more elegant. It is a silly desire.

Oh. See, when my branch of engineering came upon a problem that elegant math couldn't solve, we invented a whole new branch of math to solve it.. And then we reforged our souls so that that math ran through them and all things are that math unto us. Bow before the might of Fourier! Ia! Ia! Convolve the unbelievers!

Yes, I'm being silly. I'm not even sure how you could convolve a person. I suppose it is like taking the fourier transform of a cat

[Lvl 2 Expert]

Oh. See, when my branch of engineering came upon a problem that elegant math couldn't solve, we invented a whole new branch of math to solve it.. And then we reforged our souls so that that math ran through them and all things are that math unto us. Bow before the might of Fourier! Ia! Ia! Convolve the unbelievers!

Fourier transforms, my only weakness and nemesis! Burn them all! Wipe them from your hard drives! No more fourier transforms!

[Peelee]

Oh. See, when my branch of engineering came upon a problem that elegant math couldn't solve, we invented a whole new branch of math to solve it..

Oh, like how when Newton was having problems inventing physics, he went ahead and invented calculus to help himself out?

Fourier transforms, my only weakness and nemesis! Burn them all! Wipe them from your hard drives! No more fourier transforms!

Never had a problem with fouriers myself.

[Bucky]

My understanding is that you can do weird things to a car if you start applying the brake and accelerator at the same time. Not bad, but weird.

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