Acceleration of a car and jet plane problem

[Dale]

Please don't tell me that average 100Nm between 2000-4000RPM (from 50-70km\h) will accelerate car faster than average 50Nm between 8000-10000RPM (from 50-70km\h)

Why not. That is exactly right. A higher average torque directly means a higher average acceleration. Note, I am talking about torque at the wheels, not torque at the engine.

Edit: and I am assuming the RPM is the engine RPM and the mass and wheel size is identical. If you intended something different then my answer would be different

 

[jim hardy]

You started this thread a few days ago
https://www.physicsforums.com/threads/is-engine-torque-a-static-force.959297/#post-6083015
and were shown the error of your ways

But you are trying to hold to to your old mistaken preconceived word salad ideas. Forget them they're only confusing you. And a lot of other car enthusiasts too.
Here's what Abbe de Condillac said about such thinking about 250 years ago:

"Instead of applying observation to the things we wished to know, we have chosen rather to imagine them. Advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. These errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. The method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning. When matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my Lord Bacon says, to frame the human understanding anew.

In an automobile you feel acceleration which is in direct proportion to engine torque, (of course multiplied by gear ratios and wheel radius, and both of those are constant so long as you stay in any particular gear...and don't have a torque converter ahead of the gearbox )

Power is NOT in proportion to torque , it's in proportion to PRODUCT of Torque and RPM.
Horsepower = 2π X Torque_ft-lbs X RPM / 33,000

PS
That Condillac quote comes from a good introduction to straight thinking by Lavoisier. It's at https://web.lemoyne.edu/giunta/lavpref.html , and is worth digesting.old jim

 

[jim hardy]

Power is a RATE of doing work. So it inherently includes a unit of time. Something per second or minute, see horsepower equation above.
Torque is not a rate it's just a force albeit a force of rotation not straight line pull. No time necessary .

 

[jim hardy]

And what proffesional scientific literature You suggest me to learn about power, torque force etc?

start here ?
http://hyperphysics.phy-astr.gsu.edu/hbase/force.html
Search on words you do not understand, and even those you think you do.

 

[jack action]

In an automobile you feel acceleration which is in direct proportion to engine torque

This is wrong. The acceleration is in direct proportion to wheel torque. The difference is rather important in this case.

Please don't tell me that average 100Nm between 2000-4000RPM (from 50-70km\h) will accelerate car faster than average 50Nm between 8000-10000RPM (from 50-70km\h)

Why not. That is exactly right. A higher average torque directly means a higher average acceleration. Note, I am talking about torque at the wheels, not torque at the engine.

Why not? Because the vehicle goes at the same speed in both cases (The OP did take the time to mention it). This means that the wheel torque will be 1.5 times higher [= (50 * 9000) / (100 * 3000)] in the first case compare to the other, which is exactly comparing power outputs. This shows that a vehicle's wheel torque is proportional to power when compared at the same car velocity (as long as there is a velocity, i.e. $v \neq 0$). Your first 2 sentences are in direct contradiction with your last sentence.

For any advisors who would like to continue debating on this thread, I'm suggesting reading this https://www.physicsforums.com/threads/answering-simple-questions.953955/. At this point, the answers given are not helpful at all. They bring up confusion and frustration. Also, here is a little reminder of what can be found on PF home page (my emphasis in green text):

The Physics Forums Way
We Value Quality
• Topics based on mainstream science
• Proper English grammar and correct spelling
We Value Civility
• Positive and compassionate attitudes
• Patience and diplomacy while debating
We Value Productivity
• Disciplined to remain on-topic
• Honest recognition of own weaknesses
• Solo and cooperative problem solving

 

[Dale]

This is wrong. The acceleration is in direct proportion to wheel torque. The difference is rather important in this case.

Yes! I agree. Torque is not a conserved quantity, and gearing ratios can dramatically change the torque from the engine to the wheel. Power on the other hand is conserved (energy rather) so gearing ratios only change power indirectly through making the engine more or less efficient (neglecting small mechanical losses).

Your first 2 sentences are in direct contradiction with your last sentence

It isn't a contradiction. I specified wheel torque. He was unclear.

 

[jack action]

It isn't a contradiction. I specified wheel torque. He was unclear.

Even if he was specifying wheel torque & rpm, the fact that both cases have the same linear velocity means that the wheel radii are different, thus the forces at the wheel contact patch are still in proportion to the power:
$$P_{out} = P_{in}$$
$$Fv = T \omega$$
$$F = \frac{T\omega}{v}$$
Case 1:
$$F_1 = \frac{T\omega}{v} = \frac{50\ N.m \times 9000\ rpm}{60\ km/h} = 7500 \frac{N.m.rpm}{km/h}$$
Case 2:
$$F_2 = \frac{T\omega}{v} = \frac{100\ N.m \times 3000\ rpm}{60\ km/h} = 5000 \frac{N.m.rpm}{km/h}$$
[Too lazy to do unit conversions ]

Still in such case, the propulsive force is 1.5 times larger.

I'm specifying that because conservation of energy plays a major role here. In the debate «power vs torque», it is the great forgotten on the «torque» side. They understand Newton's 2nd law but refuse to acknowledge the principle of conservation of energy. I don't want to give them more ammunition when they'll read this thread and say: "Scientists on PF say only torque matters in acceleration, power is meaningless".

It must be clear that this is not up for debate and that it is based on fundamental scientific principles.

 

[Dale]

Even if he was specifying wheel torque & rpm, the fact that both cases have the same linear velocity means that the wheel radii are different

Oops, you are right, good point here. I was also not clear. I was assuming identical vehicle mass and wheel size, hence the specifications would be wheel torque and engine RPM.

I believe that most of the problems in the discussion come from lack of clarity. Engine vs wheel. Average vs peak vs max. Comparison criteria, etc.

They understand Newton's 2nd law but refuse to acknowledge the principle of conservation of energy

Energy is conserved, certainly. But that is not the question here. The question is about acceleration, and at all times for a given vehicle acceleration is directly proportional to wheel torque, not power. Conservation of energy is a great principle, but not the relevant one for the specific question of acceleration.

I have already explained the clear contradiction at rest, but even at high speed there is another clear contradiction. At high speeds (but neglecting dissipation) assuming a continuously variable transmission you could accelerate at constant power. In doing so your acceleration would decrease. Your power would be constant but your wheel torque would decrease, in exact proportion to the decreasing acceleration.

My assertion that torque (wheel) causes acceleration is correct, and in no way should be misconstrued to indicate that energy is not conserved. Power is important for acceleration, but only indirectly as when the maximum available power is what limits instantaneous wheel torque.

 

[cjl]

It's important to note though that at all times at a given vehicle speed, acceleration is directly proportional to horsepower. Wheel torque is almost never directly measured or referenced when discussing cars, and discussions like this lead to misconceptions like the very common believe that engine torque matters more than horsepower.

 

[Dale]

It's important to note though that at all times at a given vehicle speed, acceleration is directly proportional to horsepower.

Not at rest. From a stand-still power is 0 regardless of acceleration.

Wheel torque is almost never directly measured

Yes, that is a problem. Also, there is often a confusion about instantaneous torque vs max torque.

discussions like this lead to misconceptions like the very common believe that engine torque matters more than horsepower.

But the alternative leads to misconceptions on the physics as demonstrated by the OP. Here I am concerned about fostering correct physics much more than fostering correct purchasing decisions.

 

[cjl]

Not at rest. From a stand-still power is 0 regardless of acceleration.

Yes, but acceleration from a standstill is a very small subset of the "how fast will my car accelerate" case, and is complicated by a number of other factors.

But the alternative leads to misconceptions on the physics as demonstrated by the OP. Here I am concerned about fostering correct physics much more than fostering correct purchasing decisions.

I feel like the constant "no, power doesn't determine acceleration" in this thread is leading to far more misconceptions about physics than my statement above. In addition, knowing that it's proportional to power is fundamentally more useful, since a car's power is easily determined from its specifications and is independent of things such as driveline configuration and gear ratio. It's much simpler for a driver to know that to maximize acceleration, they must simply select whatever gear puts the engine closest to max power rather than suggesting that they must calculate wheel torque based on varying gear ratios and engine RPM.

For an example of how this can be misleading, you very confidently stated that the statement in the video in post #8 was wrong. You are wrong about this. The video is absolutely correct. At best, your statement is extremely misleading, since the video was quite clear about its statement that horsepower determines rate of acceleration (correct), and acceleration determines the reaction force between you and the seat of the car (also correct).

Fact: At all nonzero speeds, acceleration is directly proportional to power to weight ratio, and inversely proportional to speed

 

[Dale]

knowing that it's proportional to power is fundamentally more useful

Except that it is not proportional to power, it is proportional to power divided by speed (ie torque) and speed changes during acceleration. So stating that acceleration is proportional to power is kind of dodgy since the constant of proportionality is not constant. That isn’t the usual relationship meant by “proportional”.

At all nonzero speeds, acceleration is directly proportional to power to weight ratio, and inversely proportional to speed

Sure, because power divided by speed is directly proportional to wheel torque.

Look, I am not familiar with this apparently heated debate. I have no stake in it and I am just teaching physics. The power people have apparently convinced the OP of some incorrect physics as you can see from the OP. So overemphasizing power is not the be-all end-all solution.

Instead of having a power vs torque argument, wouldn't it be better to actually teach the physics of how power and wheel torque are not independent concepts but are closely related? Then you can show them how to calculate torque from power and thus figure out the acceleration.

 

[jim hardy]

For any advisors who would like to continue debating on this thread, I'm suggesting reading this https://www.physicsforums.com/threads/answering-simple-questions.953955/. At this point, the answers given are not helpful at all. They bring up confusion and frustration. Also, here is a little reminder of what can be found on PF home page (my emphasis in green text):

If i was uncivil i hereby extend my apology
was trying to be a bit forceful to break down what i perceived as a mental barrier , that's all

old jim

 

[jack action]

At high speeds (but neglecting dissipation) assuming a continuously variable transmission you could accelerate at constant power. In doing so your acceleration would decrease. Your power would be constant but your wheel torque would decrease, in exact proportion to the decreasing acceleration.

Yes, but if you double that constant power, you will double the acceleration at any speed. And this is the only way you can achieve that.

Look, I am not familiar with this apparently heated debate. I have no stake in it and I am just teaching physics. The power people have apparently convinced the OP of some incorrect physics as you can see from the OP. So overemphasizing power is not the be-all end-all solution.

Instead of having a power vs torque argument, wouldn't it be better to actually teach the physics of how power and wheel torque are not independent concepts but are closely related? Then you can show them how to calculate torque from power and thus figure out the acceleration.

I see the problem now from your perspective. So you are unfamiliar with the problem and don't see how it is to "teach" a «torque» person about the importance of power in vehicle acceleration? Let me get you up to speed with this thread. The fun starts with post #15 with OldYat47 (and, yes, it goes all the way to post #100). This type of discussion has been going on forever in the automotive community and the web forums are filled with those. You might appreciate more why you must be careful about the wording you choose when entering this debate.

 

[Dale]

Yes, but if you double that constant power, you will double the acceleration at any speed. And this is the only way you can achieve that.

Agreed, because doubling the power will double the torque. There is no power vs torque objection there.

So you are unfamiliar with the problem and don't see how it is to "teach" a «torque» person about the importance of power in vehicle acceleration?

Having “torque” people and “power” people is silly. They are not independent quantities.

 

[cjl]

I see the problem now from your perspective. So you are unfamiliar with the problem and don't see how it is to "teach" a «torque» person about the importance of power in vehicle acceleration? Let me get you up to speed with this thread. The fun starts with post #15 with OldYat47 (and, yes, it goes all the way to post #100). This type of discussion has been going on forever in the automotive community and the web forums are filled with those. You might appreciate more why you must be careful about the wording you choose when entering this debate.

Huh. I'd forgotten about that thread. I was expecting this one

 

[Baluncore]

The flow of energy is what it is all about. That is where the money is.
Energy is conserved and must be accounted and paid for.
Power is the rate of flow of energy.

Given a long enough lever and a fulcrum to rest it on, you can apply as much force as you want to a mass, only you will be limited by the distance you can move that mass.
Given the right gearbox, you can have as much torque as you want, but your speed will be limited because power (= energy flow), is determined by the engine's capacity to transform electrical or chemical energy into mechanical energy.
Whenever torque is mentioned there must be an RPM specified before energy or power can be considered, or fairly accounted for.

If engine torque is flat across a wide engine speed range then power increases with the engine RPM because power = torque * angular velocity.
If a mass is not free to move there will be no energy transferred to that mass by a force acting on it. If a shaft is not free to rotate there will be no energy transferred along that shaft by a torque acting on it. A static torque is sometimes called a moment, physicists and engineers differ in terminology there.

 

[Dale]

you can have as much torque as you want, but your speed will be limited because power

The question wasn’t about speed, it was about acceleration.

 

[jim hardy]

To return to the vernacular of car-guy circles,
(where we all like short explanations)
...
do these two statements paint a more concise yet still accurate word picture?

TORQUE determines acceleration.
POWER determines to what speed that acceleration can be sustained.

keep in mind that
in car-speak, torque and power are both functions of RPM that are nonlinear.
while
In physics-speak we like to hold one of them constant and focus on the other two as a simple ratio..

Perhaps that's been the origin of some discord in the thread?

Please correct or improve my word-picture...

old jim

 

[Dale]

Let me get you up to speed with this thread.

I really like this plot you posted in that thread:

View attachment 187751

Note that there is one unique point of peak wheel torque corresponding to the one unique peak acceleration. There are four points of peak power, none of which are the peak acceleration.

Any justification you can give as to why power should be considered as being proportional to acceleration are just torque expressed in terms of power. In any way that wheel torque differs from power, acceleration follows torque not power.

 

[russ_watters]

I didn't read all the recent discussion, but the bottom line for me is this: The videos led the OP to or reinforced in the OP a very, very wrong understanding of physics. He actually said force doesn't cause acceleration! (and in a PM to me said this even while citing f=ma). How can we endorse any explanation that leads to such a fundamental/basic wrong?

I really don't think it should be difficult to teach this correctly. The problem with the videos is they skip steps. One even had a long equation for calculating power with all the necessary elements, but without really explaining them, particular the starting point. So here's how I'd work through the explanation of power and torque in cars:

1. Linear force between the ground and wheels causes a car to accelerate. (I watched two videos and neither stated this, though one had it embedded in an equation.)
2. Linear force between the ground and wheels is generated by torque at the wheels. (Not stated or confused between wheel and engine torque.)
3. Power is torque times speed (linear or rotational), so:
a. As a car accelerates at constant torque, power increases proportional to speed.
b. As a car accelerates at constant power, torque decreases proportional to speed.

That's it. Now, this real-world torque vs power thing is based on a bunch of sloppily defined scenarios and other obfuscations. Peak vs operating points, "all else being equal" except that oops we didn't mention gearing, acceleration measured as time to a certain speed, etc. In terms of the physics, torque and power go hand-in-hand, so there is nothing to debate! The only way for torque to be different given the same power is for an accidental or purposeful deception regarding other variables.

 

[russ_watters]

do these two statements paint a more concise yet still accurate word picture?

TORQUE determines acceleration.
POWER determines to what speed that acceleration can be sustained.

More or less. But per my previous, I would prefer hammering on the fact that they really aren't separate things, but are merely two components of the same state. E.G., a car at X speed and Y acceleration has A horsepower and B torque being delivered to the wheels.

For real-world car performance, the only real caveat I'd add (probably should have in my last post) is that at the engine, if you have higher rpm and lower torque to generate the same horsepower, theoretically they are identical in performance, but in real life the lower rpm and higher torque is probably better because it should have lower losses, less rotational inertia, etc.

 

[Dale]

The videos led the OP to or reinforced in the OP a very, very wrong understanding of physics. He actually said force doesn't cause acceleration!

Exactly!

If you want to teach the importance of power, that is fine. But it must not lead to a result like this.

 

[jack action]

Note that there is one unique point of peak wheel torque corresponding to the one unique peak acceleration.

Sadly, it is not the "peak" wheel torque. It is in that gear ratio. You could have a lower gear ratio and get a higher peak wheel torque. Even at that same exact velocity, you can reach a higher wheel torque - the highest possible - which, guess what, correspond to the peak wheel power! (more details in this post)

This particular example has different gear ratios. But what if you want to compare an electric motor with a combustion engine? Or even if you want to verify a claim from someone presenting a new kind of motor? Which one has more potential to give the largest acceleration over a given speed range? It's the maximum power output you can maintain in that speed range that really tells everything you need to know. The only way it wouldn't matter is if you power a vehicle with something like gravity, where the force is constant and not limited power-wise. Although, note-worthy, it is a very unusual case that doesn't relate to most machines we know, especially cars. When someone ask about cars, I don't think assuming a power-limited machine is too big of an assumption.

The real "torque vs power" problem is «Should you maintain your engine at peak torque or at peak power to get the maximum acceleration throughout a given speed range?» The answer to that question should be unequivocally "peak power". For some unknown reasons, there are still people claiming "peak torque", mostly because $F = ma$ and there is no power in that equation.

If you want to teach the importance of power, that is fine. But it must not lead to a result like this.

Agreed. It must be corrected. Correcting is not only saying «You are wrong», but also explaining the proper way, given the sought information. I'm sure you mean well but the posts you have written that I dislike were posts #28 and #32. These posts are elitists, add nothing to the discussion and just throw every other insights out the window. They obviously left the OP confused and frustrated (Except for obvious crackpot theories, that is usually a sign that the posts are not answering the question, even though they are technically correct). This is not good for bringing people to the scientific ways. There is a context here (subject, education level, language barrier) since post #1 and the answers must be in line with this context.

but in real life the lower rpm and higher torque is probably better because it should have lower losses, less rotational inertia, etc.

In real life, the low-rpm engine will be much bigger than a high-rpm engine; there is no way around it. So the gain on the inertia point of view might not be as good as one might think. FWIW, for the last hundred years, it seems that all engine designs tends to evolve while favoring low torque and high rpm features, i.e. smaller engines.

 

[Dale]

Sadly, it is not the "peak" wheel torque. It is in that gear ratio. You could have a lower gear ratio and get a higher peak wheel torque

But that vehicle does not have a lower gear. For that vehicle that is the peak torque and the peak acceleration occurs at the peak torque not at peak power.

This is your own example, surely you don’t think it is a bad or unrealistic example. So then, without changing the configuration, just looking at that plot as is, where does the peak acceleration occur? Is it at the peak torque or the peak power? If it is at peak power then which of the four peak power points has the highest acceleration and why?

Do you believe that in all engines the peak power occurs at the same point as the peak wheel torque? If not, then for those engines does peak acceleration occur at peak torque or at peak power?

Which one has more potential to give the largest acceleration over a given speed range? It's the maximum power output

Yes, that is the one with the largest potential acceleration. And which one has the largest actual acceleration? It is the one with the largest wheel torque.

 

[vysqn]

The videos led the OP to or reinforced in the OP a very, very wrong understanding of physics. He actually said force doesn't cause acceleration!

I admit, I was wrong, I didn't understand the basics.
That's why I came here to learn and understand more of basic physics.
I have read all posts and what I've learned (I think) is this:

Force causes the acceleration of a mass because F = m*a
Engine power generate a force which accelerate a car, engine changes rotation, into torque at the wheels thus causing forward movement. More power = more force pushing car forward.
Constant engine torque means constant acceleration because power increases proportional to speed, Constant engine power means decreasing proportional to speed - if there is no counter forces like air resistance etc in both cases.
If I in my car I want maximum acceleration throughout a given speed range I have to be at maximimum engine power which generate maximum wheel torque and as a consequences - fastest acceleration

The acceleration is NOT direct proportion to engine torque, is direct proportion to wheel torque.

If Jet engine produce constant trust that means constant acceleration (If mass doesn't change and there is no air resistance)
But there is a point when air resistance is so big that plane doesn't accelerate any more and that is top speed of the plane.
We can calculate power if we want from anything which produce force because P =F*vI have one more question:
If car or plane or any object stays in motion with the same speed and in the same direction it uses Newton 1st law of motion.
If car or plane or any object start to accelerate it uses Newton 2nd and 3rd law of motion.
Correct?

BTW - sorry for my english

 

[Dale]

I admit, I was wrong, I didn't understand the basics.
That's why I came here to learn and understand more of basic physics.
I have read all posts and what I've learned (I think) is this:

Force causes the acceleration of a mass because F = m*a
Engine power generate a force which accelerate a car, engine changes rotation, into torque at the wheels thus causing forward movement. More power = more force pushing car forward.
Constant engine torque means constant acceleration because power increases proportional to speed, Constant engine power means decreasing proportional to speed - if there is no counter forces like air resistance etc in both cases.
If I in my car I want maximum acceleration throughout a given speed range I have to be at maximimum engine power which generate maximum wheel torque and as a consequences - fastest acceleration

The acceleration is NOT direct proportion to engine torque, is direct proportion to wheel torque.

If Jet engine produce constant trust that means constant acceleration (If mass doesn't change and there is no air resistance)
But there is a point when air resistance is so big that plane doesn't accelerate any more and that is top speed of the plane.
We can calculate power if we want from anything which produce force because P =F*v

Awesome! I would say that your knowledge of physics now exceeds my knowledge of cars!

I have one more question:
If car or plane or any object stays in motion with the same speed and in the same direction it uses Newton 1st law of motion.
If car or plane or any object start to accelerate it uses Newton 2nd and 3rd law of motion.
Correct?

Yes, although Newton’s 3rd law applies any time there is a force regardless of whether there is acceleration or not.

 

[jim hardy]

The acceleration is NOT direct proportion to engine torque, is direct proportion to wheel torque.

Bear in mind that the gear train connecting engine and wheels makes those two torques proportional by the gear ratio. .
By "direct proportion " i did not mean a one-to-one proportion.
My mistake, i mixed four thoughts in one sentence.
It started out okay but i appended a 'run-on sentence fragment' in parentheses. .
Here it is:

In an automobile you feel acceleration which is in direct proportion to engine torque, (of course multiplied by gear ratios and wheel radius, and both of those are constant so long as you stay in any particular gear...and don't have a torque converter ahead of the gearbox )

That's the danger of run-on sentences, they mix thoughts which leads to thoughts getting overlooked.

Excruciating attention to detail, both technical and grammatical, is the price of clear communication.
I consciously strive to write one thought per sentence and one sentence per line.
Sometimes i forget to do that.
Mea Culpa.

Torque converters add a level of complexity.
They're not quite a CVT but are a step toward one.
They transmit torque according to the difference between input and output RPM's and can actually increase torque .
That's why you'll find a car with an automatic transmission almost always has a different drive axle ratio than the same model equipped with a manual transmission.
(And it's why i put that small print caveat at the end of my run-on sentence fragment.)

Sorry for any confusion i caused.

old jim

 

[jim hardy]

My intent with this post is just to encourage simple stepwise thinking. I only want to address the real basic physics. I'm no race car expert.

Our brains don't do well in a thought experiment when we try to move multiple inter-related variables by ourself.
A fellow needs to boil the problem down to one variable that he controls, and see what Mother Nature does to the others.
We call the variable that's under our direct control the "Independent Variable" .
We vary it to stimulate the system under study and observe what the other, related variables do.
We call those related variables the "Dependent" variables.
I was taught to always plot independent variable on the horizontal axis and dependent ones vertically.

So, removing gear ratios and vehicle differences from the discussion,
in other words
boiling our thought experiment down to just one independent variable, RPM ,
and looking at a real speed-torque curve to see how torque and power and acceleration behave,
i posit:
neglecting rolling resistance, drivetrain friction, wind resistance,
and driving on level road that's neither uphill nor down
and staying in same gear !
physics tells us that

(red line is torque, blue is power..)
Acceleration will follow the red line.
because wheel torque is engine torque X the gear ratio which we're not changing for this thought experiment
and wheel tractive force pushing the car is wheel torque / wheel radius.At 2800 rpm, horsepower is 2π X 750 ft-lbs X 2800 rpm / 33,000 = 399.8 hp
At 1600 rpm, horsepower is 2π X 800 ft-lbs X 1600 rpm / 33,000 = 243.7 hp
which will be in the exact same ratio as the vehicle speeds at those RPM's (EDIT OOPS MIstake ! ) , as
make that line read ....
which will be in the exact same ratio as the power calculated by vehicle speed X wheel tractive force at those RPM's , as
@jack action pointed out earlier in his post number 42
where he pointed out OP's error of neglecting mechanical advantage.

========BORING ANECDOTE #√³∞
The fact is if you're going to drag race,
an accelerometer on the dashboard will do you more good than a tachometer.
Back in my high school days, early 60's, a good friend installed one in his hotrod.
He soon learned the "feel" of shifting after the torque peak,
and how far back along the torque curve each upshift would take him.
He optimized his acceleration over the thirteen or fourteen seconds it took him to go ¼ mile.

END BORING ANECDOTE============

now i'll try to blend basic physics with real world and my friend's experience
because i think the troubles in the thread stem partly from lack of clarity on relation between torque and power,
and partly from less than full awareness of the integral relations between acceleration speed and distance.

Acceleration is more important early in the race than late because
you need to build up speed early
so as to eat up distance quickly not waste seconds
after all you got to integrate acceleration to get speed and you got to integrate again to get distance;
and ∫(a big number) grows faster than ∫(a little number);
and you want that integral to reach ¼ mile ASAP. .

Next -- since Power is torque X (gear ratios et al) X speed
and since speed is less early in the run,
at maximum torque power is less there too. see the curve

Which takes us back to jack's conservation of energy.
Work done on the car during the race W is F X D (eq 0)
and neglecting friction&wind
it'll be all kinetic energy at the end of the track
so W = ½mv² (eq 1)
and if F = m X a
plugging that into (eq 0)
W = m X a X D (eq 2)
m and D are constant ::: mass of car, length of dragstip
to maximize W the only thing you can control is a and that's torque at the wheelsand for a sanity cross check,
going back to algebra
and equating eq's 1 and 2
W = ½mv² = m X a X D
v = √ (2a/D)
So, to maximize speed through the traps you maximize a and that's torque at the wheels..

...................

All that said,

@jack action quite beautifully explained the effect of adding multiple gear ratios to our thought experiment in that old thread he referenced.
It makes clear how and why you pick your shift points above the engine's peak torque rpm , and even above its power peak, to maximize torque at the wheels.
https://www.physicsforums.com/threa...from-engine-torque.870677/page-3#post-5482482
He plotted torque at the wheels which is engine torque X gear ratio, so his chart has a curve for each gear
worth reading...

shifting gears jumps you from one curve to the next.
i'd venture he doesn't upshift until wheel torque of the gear he's in falls below the maximum he can get from the next gear.
And an accelerometer on the dashboard will show him that.

Any help or just muddying ?old jim.

 

[Baluncore]

The question wasn’t about speed, it was about acceleration.

I am sorry I mentioned speed in post #52. I was under the impression that acceleration was the rate of change of speed.

A vehicle is not designed to be a flywheel. Torque is only being used to transmit energy from the engine to the driving wheels, where the rubber meets the road and torque is converted from a rotational to a linear force by the wheel radius. That is where the mass of the vehicle and F = m*a can finally come into play. The gearbox and differential ratios simply adjust the torque to RPM matching ratio in the drive-line. Torque * RPM will always be the rate of energy transmission = power.

A real vehicle is not a virtual mass sliding in a vacuum across a friction-free plane surface. Neither torque nor power implies acceleration of a real vehicle. Energy can flow to change the kinetic or the potential energy, or to losses. Climbing a hill requires both RPM and torque to transfer the energy needed to increase the potential energy of the vehicle as it climbs to a greater height. Aerodynamic drag, or a headwind also require torque just to maintain velocity without acceleration.

It is better with laymen, in almost every case, to consider “energy flow” in joules/sec = watts, than it is to involve the often misunderstood term torque. “Energy flow” may often be a better term than power as it is less ambiguous and more meaningful to a layman. The management and transmission of energy is fundamental to all engineering.

 

[Dale]

I was under the impression that acceleration was the rate of change of speed.

Precisely. It is the rate of change of speed, not speed. The two quantities are orthogonal.

A real vehicle is not a virtual mass sliding in a vacuum across a friction-free plane surface. Neither torque nor power implies acceleration of a real vehicle.

Good point. But in all cases the wheel torque is the mechanical quantity most directly related to the net force and therefore to acceleration.

 

[Baluncore]

But in all cases the wheel torque is the mechanical quantity most directly related to the net force and therefore to acceleration.

Only for you.

 

[jim hardy]

It is better with laymen, in almost every case, to consider “energy flow” in joules/sec = watts, than it is to involve the often misunderstood term torque. “Energy flow” may often be a better term than power as it is less ambiguous and more meaningful to a layman. The management and transmission of energy is fundamental to all engineering.

Now we're talking about how to teach.? That's subjective.

Everybody has his own life experiences that affected his thought processes.
My own concepts of force momentum and torque were formed at about age 10 to 12, pushing boats around at a marina.
That really nailed the sensory 'feelings'
and when i took high school physics, learning the names ascribed to those already familiar concepts was immediate and pleasurable.
My feel for momentum got a name mv, and that energy was ½mv² was easy enough to remember -- we didnt integrate in my high school.

so for this old layman the torque approach works just fine and is intuitive.

i think were i teaching gearheads, which is an affectionate not a derogatory term for laymen,
i'd use the torque approach and relate it to their childhood bicycling experience.
We can feel a force and estimate its magnitude
but it's a bigger mental step to sense energy with our muscles.

you notice i used your energy method as my sanity check. I'm so mistake-prone i have to cross check myself all the time.and i mistrust my math until I've cross checked two or three calculations.

Teachers should encourage students to apply physics to their everyday experiences, and vice versa. Getting one's intuition aligned with his math saves a lot of rote memorization.

just my two cents

old jim.

 

[Adam Talman]

I suppose it depends on what language and framework you are working under. At the atomic level you need the energy and collisions from atoms in order to generate a force. Therefore, if you have energy from the atoms bumping you have power producing the force. In the case of engines it may be considered the other way around. Engines produce power that accelerates the vehicle. However, I agree with Dale, I am pretty sure it is the force that you feel. My intuition tells me that is the case.

 

[Mandy D]

I realize that I am late to the party, but my thoughts on this are that I would rather consider tractive effort (force) than torque. It more directly relates to what is happening to the vehicle as a whole. However, I agree that torque is more relevant to the calculations related to acceleration than is power. I frequently have this argument with lads driving sports cars who think power is more important. My proof is to pitch my Land Rover against their sports car, select low first and floor the accelerator. It almost does not matter how much power they have (except perhaps in the extreme) against my miserable 101 kW (136 BHP) they cannot match my initial acceleration. Girls 1, Boys nil. OK, so they catch up and pass me - eventually.

F = ma
P = Tw
T2 = TR
F = T2/r

F = accelerating force
m = mass of car
a = acceleration of car
T = torque at flywheel
T2 = torque at wheels
R = gear ratio (total, main box x transfer box x axle ratios)
r = rolling radius of wheel

Calculate F from T2/r, plug back into F = ma, et voila we have an acceleration. I hope I have made no mistakes, here. :)