Acceleration of a car and jet plane problem

[vysqn]

As we know, POWER is what accelerate a car, FORCE itself doesn't accelerate a car, if FORCE cousing movement it change into work, and work over time = POWER.

The FORCE from jet engine pushes a plane forward. But as we know (or I'am wrong) FORCE don't push enything, only POWER couses acceleration. This is why I'am confused

Engine power from the car goes to the gearbox etc... and finally to the wheels.
The wheels begin to rotate, and this rotation (acceleration) is cousing by TORQUE at the wheels or POWER at the weels (i think power)?

What is correct and what is wrong with what I write?

 

[berkeman]

only POWER couses acceleration.

But F = ma...

 

[vysqn]

But F = ma...

So F = ma works in jet engine but why we don't use it at car engine? - and we combine "ma with distance over time" to calculate power ?

 

[jack action]

The force causes the acceleration ($F = ma$);
The power determines the speed you can achieve with the given force ($P = Fv$).

So, if you want to keep a constant acceleration, as the velocity increases, you will have to increase the power ($P=mav$).

Usually, you have a limited amount of power (i.e. a limited mass fuel rate to burn), so the available acceleration will decrease as the velocity increases.

 

[berkeman]

So F = ma works in jet engine

I think it's more accurate to look at the momentum of the exhaust from the jet or rocket to calculate the "thrust". What links have you been reading about how jets and rockets work? Have you read about Specific Impulse yet?

 

[Dale]

As we know, POWER is what accelerate a car, FORCE itself doesn't accelerate a car

As was pointed out by @berkeman and @jack action this is incorrect. It is the force that causes the acceleration by F=ma, not the power. For instance, when accelerating from rest the acceleration is non zero and the force is non zero, but the power is zero. If it were power that caused acceleration then nothing at rest could ever start moving.

 

[Rive]

So F = ma works in jet engine but why we don't use it at car engine? - and we combine "ma with distance over time" to calculate power ?

It depends on the actual problem you want to solve that what tool is practical. If you do it right then calculations with power might represent a shortcut when force will depend on speed, for example (for most vehicles, it does and it is quite complicated, actually). Yet, it is still force what causes acceleration - by calculating with power you just skipping some complicated math.

 

[vysqn]

At 20:56 this guy talk about what we feel when we accelerate a car. He said that power cause acceleration not moment of force (torque)... Goshh I got so little brain to understand this ...:)

 

[CWatters]

What you feel is the reaction force from the seat due to acceleration. What causes that accelerstion is the net force or torque acting on the car.

He goes further to say that it's the power that determines the torque so you are really feeling the power. That's true but don't confuse power with maximium power. Petrol engines typically have discrete gear ratios so you cannot always operate the petrol engine at the right rpm to deliver max power. So torque isn't always at a maximum.

Things are slightly different for an electric motor. They can generate close to max power over the whole of the speed range including low speeds where more of that power is available to accelerate the car (less needed to overcome drag). So an electric car can sometimes accelerate faster than a petrol car even though its maximum power output is lower.

 

[Dale]

At 20:56 this guy talk about what we feel when we accelerate a car. He said that power cause acceleration not moment of force (torque)..

The guy is wrong. This is why we use the professional scientific literature as our primary source here

 

[jack action]

We can only feel forces. Our brains can interpret those as accelerations.

The reason why power is considered the source of acceleration is because power is actually the source of the force. It is fairly simple to state $a=\frac{F}{m}$ and end it there. But having an acceleration necessarily means the velocity increases. As the velocity increases, we need more power to maintain the same force ($F=\frac{P}{v}$). Otherwise - keeping the same power input - the force will drop, therefore the acceleration will decrease as well.

This is why we can state «We need power to accelerate» ($a=\frac{P}{mv}$) even though it is the force that causes acceleration. With half the power, you will get half the acceleration as the velocity increases because you will only produce half the force (comparing the motion at the same velocity). When in motion, we need power to get a force.

 

[Dale]

This is why we can state «We need power to accelerate»

Not if v=0.

You need force to accelerate, that is it. The force is provided by the torque, so it is the instantaneous torque that provides acceleration.

The problem is that people look at engine specifications and think that the max torque or the max power is the instantaneous torque or power. It is the instantaneous torque that gives acceleration at any point, but the instantaneous torque may be less than the max torque and the instantaneous torque may be limited by the max power.

 

[jack action]

Not if v=0.

Yes, but if you constantly stay at zero then you're not accelerating. As soon as you'll reach v=0⁺, power will become relevant.

 

[Dale]

Yes, but if you constantly stay at zero then you're not accelerating. As soon as you'll reach v=0⁺, power will become relevant.

Sure, but it directly disproves the claim that you "need power to accelerate". You do not because acceleration can be nonzero while power is zero.

Again, instantaneous torque is the only parameter that is directly related to the instantaneous acceleration at all times.

 

[jack action]

The claim is that you need power to get a force while in motion. And you need a force to get an acceleration.

When comparing vehicles in motion, I can assure you that the one with the greatest power can always accelerate faster than the other one. In this context [NoteToSelf]always state the context[/NoteToSelf], you need power to accelerate.

 

[russ_watters]

We need power to accelerate
...
Yes, but if you constantly stay at zero then you're not accelerating. As soon as you'll reach v=0⁺, power will become relevant.

Sure, but it directly disproves the claim that you "need power to accelerate". You do not because acceleration can be nonzero while power is zero.

Again, instantaneous torque is the only parameter that is directly related to the instantaneous acceleration at all times.

While I agree more with Dale, let me try to spin the statement in a way that creates agreement:
We need linearly increasing power to continue the same rate of acceleration (not including losses) as speed increases.

To say that in car terms: almost any car will jump off a starting block at at a high acceleration. I had a 92 horsepower coup that I could spin the tires on if I let off the clutch to fast. But in order to keep accelerating rapidly after the initial jump, you need high horsepower. Indeed, the purpose of the gearing is partly to enable constant power acceleration (except for a Tesla, which can do constant torque acceleration).

 

[Dale]

But in order to keep accelerating rapidly after the initial jump, you need high horsepower

I am ok with that, because if you want to keep accelerating then you need to maintain a high torque and the faster you go the more power is required for the same torque. It is still the instantaneous torque that causes the acceleration, but the instantaneous torque is limited by the maximum power (at high speeds) rather than the maximum torque (which is the torque limit at low speeds).

 

[Dale]

The claim is that you need power to get a force while in motion.

The claim from the video is “power is what accelerates the car so it is always the power that you feel”. That claim is wrong.

 

[jack action]

The claim from the video is “power is what accelerates the car so it is always the power that you feel”. That claim is wrong.

Agreed. Full disclosure: I didn't watch the video.

 

[vysqn]

So " always power you feel" is wrong. It should be like this:
Engine power generate a force which accelerate (and force you feel) a car because F = ma.
More power = more force pushing car forward.

Is that correct?

 

[jack action]

So " always power you feel" is wrong. It should be like this:
Engine power generate a force which accelerate (and force you feel) a car because F = ma.
More power = more force pushing car forward.

Is that correct?

Yes.

 

[AZFIREBALL]

It is all a matter of definition. You say force does not cause acceleration. This is incorrect.

Force causes the acceleration of a mass (F=ma) by definition.

Let us look at an ideal case. If you apply a force to an unrestrained object(mass), the mass will accelerate. It will not just give it velocity but, it will accelerate. This is hard for some people to see because every object they encounter is restrained to some extent by external conditions.Take the case of an object (mass) in outer space. If you apply a force to this mass, it will start to accelerate. As long as this force is applied, it will continue to accelerate. Not just move forward at a constant velocity but, accelerate (ever increasing velocity). This concept is counter intuitive to those of us who live in an environment where every object we encounter has constraints of some sort. The constraints maybe, friction, gravity, air resistance, etc. Anything that keeps it from moving (obtaining a velocity).In our environment we are required to continue to apply a force, just to keep the object moving at a constant velocity due to the constraints. To get an object to accelerate we must add additional force over and above this minimal force. We all know, to keep a car with a dead battery moving, it requires that a substantial force be continually applied to just overcome the constraints.A car’s engine changes rotation, into torque, at the wheels thus causing forward movement.

This torque is converted to a force at the axel level by the reaction of the lever-arm(radius of the wheel) pushing against the friction existing between the tire and the road surface, after all constraints have been overcome. This is the FORCE that pushes the car forward.

 

[vysqn]

Yes.

Torque from the engine is limited by revs. We can use CVT transmision to accelerate forever but there we hit a point where all forces (friction , air resistance, etc) will stop car and that's the car top speed.
Is that Correct?

So let's go back to jet plane where we can also use 2nd Newton's law.
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 using P = Fv .
Is that correct?

 

[russ_watters]

So " always power you feel" is wrong. It should be like this:
Engine power generate a force which accelerate (and force you feel) a car because F = ma.
More power = more force pushing car forward.

Is that correct?

Yes.

No! Jack, this is the argument you were having with @Dale again. The current form of the OP's statement does not mention speed. The relationship between force and power requires an unstated assumption about speed.

You may recognize the unstated assumption, but it isn't at all clear the OP does so we should not be agreeing with any answer that leaves it unstated.

 

[vysqn]

For instance, when accelerating from rest the acceleration is non zero and the force is non zero, but the power is zero. If it were power that caused acceleration then nothing at rest could ever start moving

This is start to be more confused than it was:)
So torque is moment of force, a parameter that is directly related to the instantaneous acceleration at all times
So why we even calculate power? If power doesn't generate a force which pushing car forward?
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)

 

[russ_watters]

So torque is moment of force, a parameter that is directly related to the instantaneous acceleration at all times

Right.

So why we even calculate power? If power doesn't generate a force which pushing car forward?

Because we also care about speed and fuel usage and engine displacement (size).

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)

Here you're confusing engine torque with wheel torque.

 

[vysqn]

This is Jason Fenske who is mechanical engineer said:
at 2:10 that "power is what gives you speed, what gives gives you acceleration"
and at 7:05 "how fast will a car be able to accelerate, what will its top speed be - that comes down to power"


This is John Cadogan who is also mechanical engineer and he said:
at 12:40 "power at the wheels produces force which pushes you forward making you accelerate and speed which you acquire along the way as a consequence"

So who is right...

 

[Dale]

Newton is right. Sorry, none of these videos is teaching you correct physics.

 

[vysqn]

How is that possible that those 2 enginers are wrong?

 

[russ_watters]

How is that possible that those 2 enginers are wrong?

As a fellow mechanical engineer, it's disappointing. They are being sloppy. When they say "acceleration", they are talking about an average over a variety of speeds, which is a twist on simply stated acceleration.

Sometimes the sloppiness doesn't matter, but here it lead you from a description that was a little sloppy to an understanding that was very, very wrong.

Also, my understanding is there is a furious but silly debate about torque vs power among car guys.

 

[vysqn]

Newton is right. Sorry, none of these videos is teaching you correct physics.

if it is true, it is ruining my worldview. I start to learn some physics just for fun some time ago because I was curious how torque and power works. And those guys looking trustable for me...
And now I feel crushed...

 

[Dale]

How is that possible that those 2 enginers are wrong?

Note, the requirements of PF, as I have already mentioned above, are that all posts on PF must be consistent with the professional scientific literature. That means textbooks and peer-reviewed papers. Youtube videos are not peer-reviewed and they are not vetted. They are often of low quality scientifically, as here.

I am sure that if those engineers were preparing a scientific publication then they would have been more correct in their statements. But it is not our goal to correct all youtube errors so it is pointless to send lots of videos containing such errors. We recognize that they exist and that is precisely why the acceptable standards here are higher.

 

[vysqn]

Note, the requirements of PF

What is PF ?

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

 

[jack action]

@vysqn :

I'll try to demonstrate the problem from another perspective. You start with the engine and try to figure out where the power/torque goes. It is in my opinion that it is best to look at what needs to be achieved to reach the desired objectives.

There is a vehicle (car or plane or whatever) of mass $m$, it has a velocity $v$, an acceleration $a$ and some resistance $R$ (drag, rolling resistance, etc.).

To maintain this set up, you must also have a force $F$. The force $F$ can be found with the following relationship:
$$F = R + ma$$
But, there is also the notion of power $P$ that is important. The following relationship must also be respected:
$$P=Fv$$
What does this means?

Whatever power source you have (combustion engine, jet engine, electric motor, horses or humans pulling the vehicle; anything), it must be able to produce the force $F$ and the power $P$.

Usually, the force $F$ is easy to achieve (with a gearbox for example), no matter what the power is; no matter whether this power is composed (at the source) of 'force' or 'velocity'. But power is maintained through any change (not considering minor losses) due to the principle of conservation of energy.

What happens if $a=0$? Then $F=R$ and $P=Fv$. If this is the maximum power you can produce, then $v$ is your maximum velocity.

If you can increase the power by $\Delta P$, then - at this velocity $v$ - the force $F$ will increase ($F+\Delta F=\frac{P+\Delta P}{v}$), thus the acceleration $a$ will increase ($0+\Delta a=\frac{F+\Delta F -R}{m}$ or $\Delta a=\frac{\Delta F }{m}$), which in turn will increase $v$, setting you to a new equilibrium. Note that you might need to do some transformations along the way to make sure $P=Fv$ for the vehicle (like changing the gear ratio for example).

What happen if $v=0$? Then $F=R+ma$ and $P=0$. This means that as long as the power source can produce the force $F$, you are OK. How much power does it [keyword]needs[/keyword] to produce? None.

So a 5 hp lawnmower engine can theoretically produce the necessary force to reach the same acceleration that a 10 000 hp dragster engine can produce ... at $v=0$ and only at $v=0$. Note that in reality, there might be so much losses in the transmission that the force $F$ will not be reached (In fact, either $R$ or $m$ will be greater in some way).

But if the engine produces, say, 300 hp at $v=0$, where does it all go? It goes into wasted heat (the tires spin, clutches slip, etc.), mechanical energy (stored in rotating parts because of inertia) or elastic energy (parts will deform).

What I'm saying is that instead of starting with the engine and trying to figure out what you can do with it, try to analyze what is required by the vehicle.

 

[Baluncore]

What is PF ?

You are there. PF is www.physicsforums.com

There is a different language out there where the identical words have completely different meanings.

When I pointed to the two faded blue marks on the tachometer and asked if they were the equal power points I was told; “No, those are the equal torque points”, and that “it is not power until it gets to the back wheels”. I chose NOT to argue, so the inspector granted my license upgrade.
I do know that you should change gear at equal power points, which happen to be equal torque points at the back wheels, but they are not equal engine torque points. The engine RPM and gear ratio steps have a lot to do with that.