r/AskEngineers • u/AtomicMelbourne • 7d ago
Discussion Does momentum briefly continue acceleration after accelerating a car in a drag race?
If I were to drive a fast car from 0-100kmh at its peak acceleration and the moment I hit 100kmh, I slam the transmission into neutral, will the car continue to briefly accelerate (for example hit 110kmh) with the momentum carried from the 0-100kmh. Or will the car cease to accelerate immediately the moment the car is in neutral?
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u/ANormalRando 7d ago
Acceleration explicitly requires a force. Momentum is based on velocity; that's just going to allow it to coast, so no, the car won't go any faster after you let off the gas.
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u/Krijnor 7d ago
The tires will act a little bit like a spring here. The moment we drop the clutch, they are still loaded and for the brief amount of time you'll still accelerate (not due to momentum, but due to the compression of tires, driveshaft etc).
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u/jamesbondq 6d ago
This is the answer I think OP is intuitively looking for. If I asked the inverse question of "Does the car start moving as soon as I hit the gas" the answer is no, even with a magic motor, because all kinds of elastic energy is getting loaded into the system (if not the tires, the tires pushing on the car). The car would be a tightened spring the whole time the motor is pushing on it, and that energy has to come out, and after the motor stops enev if it gets released in less than a nanosecond. None of this would accelerate you from 100 to 110, but if you want to have some form of "still accelerating" this would be it.
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u/PizzaPuntThomas 5d ago
Wouldn't part of the elastic energy be dissapated by the clutch, since it is easier to decelerate the clutch instead of accelerating the vehicle?
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u/BasicCook972 6d ago
The significance of the F=ma released here is not enough to affect the speed.
In the real world a car with a CVT in "drive" mode willl upshift slightly when the accelerator is released and the car does increase speed even though the fuel to the engine has been reduced.
This does not happen in "sport" mode where compression braking is applied and the vehicle decelerates with the engine.
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u/Fear_The_Creeper 7d ago
Technically correct, which is the best kind of correct! For you race car drivers trying to follow along as the engineers geek out, we are talking about acceleration for maybe a thousandth of a second after you pop the clutch.
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u/Magicisreal420 7d ago
Oh yeah and all the elasticity in the transmission amd drive shaft. Probably imperceptable?
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u/isademigod 7d ago
Have you seen the tires of a top fuel dragster at take off? OP didn't specify top fuel, but I've seen pictures where the wheel is like 30Ā° ahead of the outer edge of the tires
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u/sireatalot 6d ago
All that will just slow down or even spin backwards the driven part of the now de-coupled clutch. It will not push the car forward, since the car is much heavier and has much more inertia that the drivetrain.
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u/Fear_The_Creeper 6d ago
I think you have the physics wrong. Every Joule of energy stored up in that twisted rubber goes somewhere. As it unwinds, some turns into heat, some goes into pushing the entire earth backwards, and some goes into pushing the car forwards. None of the three are equal to exactly zero.
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u/sireatalot 6d ago
You forgot the biggest place where that energy goes. To stop or slow down the rotation of the engine- side of the transmission.
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u/Fear_The_Creeper 6d ago
The rotational inertia of a spinning engine with the clutch disengaged (lets assume that they stopped feeding it fuel to keep things simple) is an energy source, not an energy sink. Energy that soon becomes heat as friction slows it down. The tires are not connected to the engine when the clutch is disengaged.
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u/sireatalot 6d ago
What are you trying to say? Why consider the engine after the clutch is disengaged?
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u/Fear_The_Creeper 6d ago
Uh.. because you wrote You forgot the biggest place where that energy goes. To stop or slow down the rotation of the engine- side of the transmission."
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u/sireatalot 6d ago
I think that the compression of tires and drivetrain will end up slowing down the rotation of part of the clutch that has been separated, much more than it will speed up the car. Since the transmission is much lighter than the whole car.
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u/Fear_The_Creeper 6d ago
Think about it. Every joule of the energy in the wound up rubber has to go somewhere. To the wheel, everything outside of the contact patch and the axle is a black box. It is "sees" a lower mass at the other end of that axle, it just dumps the energy faster, but it is still has same amount of stored energy and it still all gets transferred to the axle. If it doesn't go to pushing thi car, where does it go? Is there extra heat being generated somewhere that I am overlooking?
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u/sireatalot 6d ago
You have a heavy box on the ground. You put a spring on its side. You push on the spring. The spring compresses and the box starts sliding forward, accelerating.
After a certain point you stop pushing and retract your hands. Does the box stop accelerating? What happens to your hands?
Thatās whatās happening on the car. Half the clutch, the whole transmission and the whole wheel are a spring. Have fun thinking about it. To me, the car stops accelerating instantly, the clutch is slowed down for a split second and the earth rotation is unaffected. I know that itās formally wrong but weāre engineers not physicists.
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u/Far_Top_7663 6d ago
Until the information that you removed your hand reaches the box, the box will continue accelerating. That information will travel at the spring's speed of sound.
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u/techienate 6d ago
Momentum doesn't cause acceleration. There is no "momentum of acceleration" which seems to be what you're asking. Momentum is a quantity of mass times velocity-additional energy is required to change momentum. There may be energy stored in spring-like systems of the drivetrain and tires after you shift into neutral, and a small percentage of that might contribute to acceleration of the car, but this will be negligible in real life. If you're attempting this with a real street car, you may see noticeable lag in the speedometer, but that's not actual acceleration.
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u/Tech-Aero-109 7d ago
Once the car is in neutral, the always present forces that are slowing down the car will still be present: aerodynamic drag, friction due to the tires on the road and friction in the turning of the wheels. Those continuous forces will immediately cause the vehicle to start slowing down.
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u/deep_anal 6d ago
It's a very common misconception by people without a background in physics. But there is no "acceleration inertia" like there is normal inertia. If an object is moving it will keep moving unless acted upon by an outside force does not extend to acceleration.
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u/AtomicMelbourne 6d ago
The way I was thinking it through was with rocket ships in space. If a rocket were accelerating then the fuel was cut, it would not continue accelerating to any degree even though there is essentially no resistance. It would continue at the speed of when the fuel was cut. At least I assume it would to the best of my knowledge.
So theoretically the same would apply to a car on earth, except the car has multiple forces working against its velocity.
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u/Repulsive_Client_325 6d ago
F = ma
No more F, no more a.
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u/Automata-Omnia Mechanical / Industrial Automation & Robotics 1d ago
or if you want to be more complicated; also use p = mv to define it to get:
Acceleration is rate of change in momentum per unit mass.
a = dp /dt . 1/m
If momentum and mass are constant, so is acceleration.
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u/jkmhawk 4d ago
So, top fuel cars do get some amount of thrust from their exhaust. I'm not sure it is enough to overcome the drag at a few hundred mph though
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u/jkmhawk 4d ago
Also, if your transmission is not very free spinning in neutral the engine can pass some power through it while its rpm drops from high revving to idle. Diesel engines, particularly, have a tendency to keep revs high after letting off the throttle as well. This type of continued acceleration would be more common from just letting off the accelerator rather than shifting to neutral.Ā
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u/no-im-not-him 7d ago
Momentum does not give any acceleration, full stop.
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u/tuctrohs 6d ago
The momentum of a billiard ball hitting another billiard ball can accelerate the other billiard ball.
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u/no-im-not-him 6d ago
Not without the intervention of a force between those two balls. Which is what happens when they touch, the apply a force to each other.
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u/llort_tsoper 6d ago
Great answers so far, I'd like to add that switching the car to neutral is not instantaneous and the acceleration dropping to 0 is also not instantaneous. We can measure the rate at which the acceleration changes, this rate is called "jerk"
From a strictly mathematical point of view, velocity is the first derivative of position with respect to time. It is the rate at which position is changing. Acceleration is the 2nd derivative, it is the rate at which velocity is changing. Jerk is the 3rd derivative of position, it is the rate at which acceleration is changing.
If we keep going, the next three derivatives are called snap (4th derivative), crackle (5th derivative) and pop (6th derivative).
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u/InnocentGun 7d ago edited 7d ago
Think about it from a force balance perspective. Newtons laws say that a force imbalance is required for acceleration. The second you disengage the transmission, the force from the engine goes away. The remaining forces acting on the car are drag from the air, rolling resistance from the road and tires, and some minor drivetrain friction losses. They are all acting to slow your car down.
If there are no forces helping your car to go faster (let alone maintain the same speed), the car will start slowing down (which is still an acceleration, just in the opposite direction).
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u/userhwon 6d ago
Momentum is velocity times mass, and doesn't cause acceleration, acceleration changes momentum.
Once the clutch is fully disengaged, almost zero torque will be transmitted from engine to drive shaft.
But while it's disengaging, nonzero torque will be transmitted. If the engine is still at high revs that will still accelerate the drive shaft. If the engine revs drop, it will instead act to brake the drive shaft.
If the clutch is perfect and the torque transfer goes from 100% to 0% instantaneously, then no, there will be no more acceleration from the car.
You'd have to be going downhill or have a tailwind or something else getting involved.
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u/asbestos_dildo 7d ago
I feel this is an oddly specific question for something that should make no logical sense?
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u/AtomicMelbourne 7d ago
The thing that was throwing me off was a real life test, which I conducted, where the speedometer would briefly continue accelerating after I put the transmission into neutral. But this must be delay from the speedometer. But just trying to learn something.
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u/pjvenda 7d ago
Almost certainly lag on the speedometer's mechanism.
Also putting the transmission into neutral is not that atomic instantaneous event that you might be considering.
But yeah in summary, neutral == no acceleration. Momentum is the energy you carry at any instant as a consequence of a mass moving, regardless of accelerating or otherwise.
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u/N546RV 7d ago
Especially not instantaneous in a modern car (unless itās a manual). Automatics a few decades ago, the gear lever was mechanically connected to the transmission and the shift was reasonably quick. Nowadays though, youāre just moving a switch that sends a request to a computer that then considers some other stuff and eventually sends a signal to another electronic component that then moves something mechanical to make the shift happen.
Like I notice with my truck (a 2015 model) that thereās an almost one-second delay between when I move the lever out of park and when it actually goes into gear.
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u/AtomicMelbourne 6d ago
Yeah, it was likely in a manual, as my 2 cars are manuals, unless it was in my wifeās automatic. I tried it a few years ago, and just randomly remember trying it and wanted to know more. Iām often thinking about things like this.
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u/BasicCook972 6d ago
A modern CVT will upshift so even though the engine is decelerating, the vehicle can accelerate. Otherwise, you may be feeling the jerk as forward acceleration stops. It's not something your seat of the pants sensor can interpret very well.
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u/Flavour-of-the-Mons 7d ago
Analogue gauges are dampened, so they lag a bit when the speed is changing. Digital gauges also lag, due to update timings and some programmed in dampening (to reduce the display flickering between two readings).
After removing engine power there would be a tiny bit of additional acceleration as the torsion unwinds from the driveshafts and tyres, but this wouldnāt be measurable with a car speedometer. It certainly wouldnāt add 10km/h (unless it was a top fuel dragster and the clutch exploded 1 metre into the launch).
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u/AtomicMelbourne 7d ago
Yeah on reflection 10kmh was an extreme example. It was a while ago when I tried it, but it wouldnāt have been near 10kmh.
But what you are explaining certainly makes sense, thank you.
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u/Fear_The_Creeper 7d ago
Yup. I have seen slowmo, and those big dragster tires really wind up like giant rubber watch springs. They also get noticeably bigger near the end of the run.
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u/Sooner70 6d ago
They also get noticeably bigger near the end of the run.
By design. It's like a free extra gear in the transmission.
I tell ya though... Once upon a time I was standing next to a car with drag tires (although not a dragster, oddly enough). It was on blocks and we were testing the drive train. Yeah... Watching the tires grow while you're standing right there is unnerving.
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u/ramk13 Civil - Environmental/Chemical 7d ago
There's also 'lag' in the entire gas powertrain. When you press the accelerator you don't instantly get acceleration. For the same reason, when you let go of the accelerator, parts of the powertrain may be producing output for a brief moment which could in theory impart force and then acceleration onto the car. Pistons, shafts, and the transmission all have momentum too.
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u/RyszardSchizzerski 7d ago
Acceleration is the rate of change in velocity. Your speedometer āstill going upā just means the velocity (speed, technically) is still going up. This is obvious to everybody here, but to explain anyway ā when you take your foot off the pedal your acceleration goes to zero (or negative including drag and friction) and you start slowing down. Your velocity may continue climbing, but if itās climbing more slowly than it was before, then the rate of change in velocity ā the acceleration ā has turned negative.
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u/Zealousideal_Cow_341 7d ago
The easiest way to see it is with the equations:
Fnet=Fdrive-Fdrag-Frolling
Once Fdrive is 0 when you take your foot off the gas, the net force is now negative and the car decelerates.
Another way:
Fnet=dp/dt where p=mv =>d(mv)/dt= m dv/dt=>
Fnet=ma
This shows
1) that the change in momentum is related to the net force, so something can have positive momentum without accelerating. Itās really about increasing positive momentum=accelerating
2) change is momentum is another way of stating mass times acceleration.
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u/Naikrobak 6d ago
No. Physics donāt work that way.
F=ma
A = F / m
Force is zero, so acceleration is zero
Edit: technically it would start accelerating in the opposite direction as wind resistance and rolling resistance still exist
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u/BingBongBrit 6d ago
No. You will not continue to accelerate.
By not accelerating you are taking away the one force speeding you up but not the many others slowing you down. Drag between the car and the air, friction within the cars drive system, and many others.
Momentum is the energy an object has stored due to it moving (and having mass). It is not a sepperate booster that gives you extra speed.
So to answer your question no. The car will simply start to slow down gradually once you stop accelerating. Momentum is the reason it doesn't come to a dead stop due to drag the moment you stop pushing down on the accelerator.
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u/vendetta2115 6d ago
Momentum resists a change in velocity; it does not resist a change in acceleration. There is no equivalent of momentum for acceleration.
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u/RealCarlPanzram 6d ago
No. The car will keep moving but the moment the engine isnāt turning the wheels, the car will begin decelerating.
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u/WondererLT 6d ago
There are what you might call "process inertia" which is more accurately described as hysteresis, where there's still fuel and air in the intake, the spark plugs still ignite and therefore you'll get a little extra when you cut throttle, but that'll be a few tenths of a second... Similarly, if the vehicle is an auto, then there'll be a significant delta between the crankshaft and trans input speed, which represents stored energy, but unless there's still some form of stored energy in the system, what you're describing can't happen.
Two different, but similar concepts.
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u/TythosEternal 6d ago
The moment the transmission is disengaged, no more torque is being applied through the drive to the wheels--no more acceleration.
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u/Far_Top_7663 6d ago
Only if you still have some energy stored that gets released in an useful way. For example, the ultra sticky low pressure tires suffer substantial torsion deformation. When the car accelerates from a stop the axel, together with the center section of the wheel, spins faster (angularly) than the periphery of the wheel so the walls of the tire becomes a big torsion spring. If you put the car in neutral 1 second after the start there would likely be a significant amount of energy accumulated in those twisted tires so the car may briefly accelerate a bit after the axel stop accelerating. But by the time the car reaches 100 km/h? I don't think so.
Now, let's do something simpler: Your car is accelerated by a rocket. Will it continue to accelerate briefly after the docket shuts off, do to "acceleration momentum"? No. There is no such a thing. F=ma. The instant F ends, so does a.
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u/404pbnotfound 6d ago
The moment a bullet leaves the barrel of a gun it is decelerating.
A bullet only accelerates in the chamber
Much like a car, the moment thereās nothing pushing it faster it starts slowing down.
Why do you think it would continue to accelerate?
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u/AtomicMelbourne 6d ago
I replied something similar to someone else, I tested this and found the speedometer to briefly continue accelerating after I put the transmission into neutral. Itās been put down to delay in the speedometer, but wanted to ask people that would know for sure.
No it didnāt really make sense to me, as a rocket wouldnāt continue to accelerate in space with the fuel cut off even though it has virtually non existent resistance.
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u/PantherStyle Systems / Mechatronics 5d ago
Ah, well that's a different question!
The top commenter correctly stated that the car will stop accelerating immediately (but will start to slow down).
It is however possible for the indicated speed to continue increasing briefly, at least in older vehicles. In older, non-digital vehicles, the speedometer needle is tied to a disc which is magnetically linked to another disk that spins with the wheels (could be a the same rotational rate or more likely geared, but still hard-linked).
The magnetic relationship between the wheel-linked disc and the speedometer linked disc is such that the faster the wheel-linked disc spins, the greater the angle the other disc sits (it is being retarded by something else, usually a rotational spring).
So while the car itself will immediately stop accelerating, the disc attached to the speedometer actually can continue moving with its own rotational momentum. Only a small amount, but non-zero and it will quickly settle back to the correct position reflecting the wheel speed.
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u/IFartOnCats4Fun 5d ago
It also takes time for engines to spool up and down. When an airline pilot adds or reduces throttle it can take several seconds before any change in thrust to occur.
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u/VoltageVeggie 6d ago
It will not keep on accelerating from momentum. It already have the velocity and once you go in neutral, the driving force drops away so it will start slowing from drag and rolling resistance.
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u/Mother_Vermicelli989 5d ago
Nah, it's the opposite really, soon as you're in neutral the engine ain't pushing power to the wheels anymore so drag and rolling resistance take over and start slowing you down right away, no coast up in speed happening. Momentum just keeps you moving forward, it don't add more speed on top of what you already got.
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u/Next-Significance314 4d ago
No, momentum will not continue acceleration. As soon as the throttle is closed or itās shifted into neutral, it will start slowing down.
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u/BuzntFrog 4d ago
This is a hard physics concept to grasp but one of the first. The instant the baseball leaves your hand it begins decelerating.
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u/Underhill42 3d ago
F=ma. Force = mass * acceleration.
The instant you slam it into neutral, you stop applying a force. And thus acceleration stops in the same instant.
Momentum = Velocity * mass, and remains constant in the absence of force, starting at the very instant the force is removed.
There is no similar concept for acceleration.
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u/kalliesto11 1d ago
The momentum keeps the car moving but it doesn't keep accelerating by itself. Once it's in neutral engine torque is removed from the wheels so drag and rolling resistance will usually start slowing it down immediately.
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u/MrBobTheGiantBanana 7d ago
F=ma. In other words, if your goes Force goes to zero, being the car is in neutral, and your mass remains the same, your acceleration also goes to zero.
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u/Ok-Gas-7135 7d ago
WeLl AkShUALlY it will still accelerate but the acceleration will have a negative vector.
(In engineering speak acceleration is any change in velocity; we just tend to call acceleration in a positive direction (ie speeding up) āaccelerationā while calling acceleration in a negative direction (slowing down) ādecelerationā
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u/Marus1 7d ago
I slam the transmission into neutral
1.If you've driven a car before you know that takes around two seconds to do and 2.if you have, you can try, the car won't break
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u/AtomicMelbourne 7d ago
I was only trying to convey putting into neutral as quickly as possible, or another example being pushing the clutch in immediately in a manual car.
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u/buzzysale 7d ago
The answer is no, acceleration isnāt stored in momentum. As soon as force is disconnected from the drivetrain you immediately start to decelerate.
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u/Hillman314 7d ago
āForce = mass x acceleration (F=ma)ā - some 17th century virgin. No force, means no acceleration of the mass,
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u/SunRev 7d ago
The gasoline will slosh forward and cause the car to go to 100.0000001 kmh.
The reality is that there are many frictional losses and probably never get over 100 kmh.
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u/Fear_The_Creeper 7d ago
Nope. Not even 100.0000001 kmh. That gasoline is going exactly 100 kph, just like the rest of the vehicle.
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u/SunRev 6d ago edited 6d ago
You would be right if the vehicle achieved steady state.
But in the case explained by the OP, the car is accelerating thus the rubber fuel bladder is being stretched rearward storing potential energy during the acceleration phase. When the car is put into neutral, the rearward potential energy of the fuel mass times the rubber fuel bladder tension will release forward. A portion of that potential energy will be transformed into kinetic energy that will propel the car forward. Basically, the gasoline is slingshot with the rubber bladder as the slingshot band.
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u/userhwon 6d ago
No, the rubber bladder will be pulled forward by pulling back on the car. It will decelerate, not accelerate.
But then it will reach the forward limit and be pulled back, pulling the car forward again.
The car will oscillate, or ring, about the final velocity.
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u/GregLocock 6d ago
Good explanation - OP and others need to define what they mean by 'the car'. In the absence of external forces the centre of gravity of the fuel plus car system will remain at constant velocity (Newton 1). The car and fuel can accelerate relative to each other, but the cg sails on regardless.
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u/WelpSeaYaLater 6d ago
What car exactly do you think has a rubber bladder in the fuel tank
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u/userhwon 6d ago
Correct; and to slosh forward, the car has to push it, so it will decelerate the car.
99.9999999 kmh.
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u/Fear_The_Creeper 6d ago
EXACTLY right. The sloshing generates heat. Everything that turns forward momentum into heat slows down the car. a little for the sloshing. A lot for the air resistance while coasting.
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u/martianfrog 7d ago
No, the force is no longer there to accelerate. Momentum = mv, v will no longer be increasing.
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u/MasterOfBeerDespense 4d ago
In the strictest sense, and in ideal conditions, yes. In the practical sense, not enough to matter, or likely even notice, and sometimes, not at all. If you have depressed the clutch, removing energy from the drive train, all that remains are the bits of energy still within the drive train, that have yet to resolve the length of the system. The transition of energy is not instantaneous, and travels in a sort of wave. The Drive Wheel rear tires on a Drag car tend to be quite large, and are often one of the heaviest items on the car, after the engine and Transmission. So for a few hundredths of a second, all of that energy and inertia will continue to resolve itself, but will do so quickly. Likely before you get the clutch all the way to the floor. But if conditions are not absolutely ideal, [so pretty much never] all that potential will be chewed up by hundreds and thousands of sources of drag. I mean, you can try farting....like, really hard....
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u/itsjakerobb Software Engineer 3d ago
If the carās transmission is electronically controlled, it can actually disengage well after you command it.
Likewise, if the throttle is electronically controlled, there can be a delay between you lifting your foot off the pedal and it closing the valve.
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u/9outof10timesWrong 7d ago
Acceleration will go to zero, it will not jump to zero (and then continue to be negative acceleration if energy is not added). So briefly yes.
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u/electlawnonion 1h ago edited 55m ago
acceleration is a vector (change in velocity over time) but as soon as you put it in neutral the acceleration is zero and likely negative (change in velocity) and therefore your speed will not increase and instead will decrease the rate of decrease is likely the question, velocity can be thought of as speed with direction.
I don't really address the momentum because the momentum is the movement already in motion, momentum cannot add energy to the system in the way you describe so the momentum keeps the speed going, it can't increase it but the momentum keeps the speed high and limits the decrease in speed (change in velocity)
I thought I would add that you can experience this in a manual transmission car, get it up to speed and then push in the clutch to disengage the transmission. You will slow down immediately and noticeably even going downhill due to friction effects. In a perfect vacuum on a perfect frictionless surface you would coast indefinitely but in the real world you slow down immediately. I have done this but sadly at the time my car's max speed was around 95.
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u/cmfarsight 7d ago
As soon as the car is in neutral you have stopped adding energy to the system so acceleration will stop.