Merged DDWFTTW in a “Real” Wind

[69dodge]

Did you know that the friction between car tires and the road is static friction, even while the car is moving and the wheels are rotating? As you said " has to do with the fact that the Earth exerts a force only on the part of the tire in contact with the ground, and this part of the tire is momentarily motionless."

This is the basis for anti-lock brake systems (ABS) . Since the maximum static friction force is larger than the kinetic friction force, a car can be stopped fastest if we prevent the wheels from locking up. When the wheels lock up, the friction force is changed to kinetic friction (the tires and the ground are moving with respect to each other) thereby reducing the acceleration and increasing the time and length required to bring the car to a halt.

Note that it is only when kinetic friction is present that the tires and ground are moving with respect to one another.

So far, I agree with you.

So the carters claim that the earth must be accelerated because there is a force on it from the cart is false.

I disagree here. What happened to F = ma?

Static forces do not do any work.

I know what static friction is: it's the friction between two objects that are not in relative motion. I am not sure what you mean by "static force" in general, but from your statement that static forces do no work, I am guessing that it's a force on an object which isn't in motion at all. Is that correct?

So, although both terms include the word "static", static friction might not be a static force. If both the object applying the frictional force and the object to which the frictional force is applied are moving at the same speed, then the friction is static friction, because the two objects are not in relative motion, but it is not a static force, because the object receiving the force is in motion. Such a force does do work.

In fact, what I have proposed (using the ground as the stationary reference for an inertial frame where only the cart and the wind are in motion) is such a common practice that I am amazed at the futile arguments being made against it!

It is common. And as I said here, you can do it if you want. But if you do it, you can't invoke conservation of momentum.

As I said before, 69dodge, I appreciate the open-minded approach you have to the subject. It is a breath of fresh air from the closed minded dogma of the carteers.

Thanks. I try.

 

[69dodge]

All I am attempting to establish is that the surface of the earth can be used as the basis for an inertial reference frame for both the cart and the wind, ...

It can, yes.

... and the momentum and energy of the cart and the wind must be conserved in that frame.

Their momentum is not conserved, because they are not a closed system. The Earth exerts a force on the cart, which changes its momentum from what it would be without that force.

I have been saying until now that their kinetic energy is conserved. But I forgot about all the frictional losses involved, which turn kinetic energy into heat. So, even though the Earth doesn't acquire any kinetic energy, the kinetic energy of the air and cart isn't conserved either.

 

[69dodge]

Applying a force to an object does no work if the object remains static in an inertial frame - i.e. it does not accelerate. This has nothing to do with movement between the object and whatever is applying the force, but simply to acceleration of the object.

You mean "move" here, not "accelerate".

 

[Jack by the hedge]

You mean "move" here, not "accelerate".

I think the terms would be interchangeable here. If the body is initially static then "move" is fine. In the more general case, it might already be moving. If the force does not cause the body to change its velocity (whether that's initially zero or not) then it does no work on the body.

 

[69dodge]

In the more general case, it might already be moving. If the force does not cause the body to change its velocity (whether that's initially zero or not) then it does no work on the body.

I'm pretty sure that's not right.

Suppose you push a car at constant speed, against the force of rolling resistance. This requires you to do work on the car. Do you disagree?

(The energy you expend doesn't get turned into kinetic energy of the car, because the car remains at the same speed. Instead it gets turned into heat in the tires, bearings, etc.)

 

[humber]

It would have been better still if you had bolded the important phrase: "...for an object subject to negligible external forces..."

If the retarding force applied to the cart through its wheels point of contact with the ground "does nothing to affect [your] calculations", then you are doing the wrong calculations.

Knock yourself out. Power the ground with the propeller.
Fly to the Moon.

 

[humber]

Just a few strays left to go.

 

[Jack by the hedge]

I'm pretty sure that's not right.

Suppose you push a car at constant speed, against the force of rolling resistance. This requires you to do work on the car. Do you disagree?

Fair enough. I agree with that. I was considering the difference between applying the force and not applying the force. If initially applying the force doesn't cause acceleration then etc.

 

[Jack by the hedge]

Just a few strays left to go.

So far there's you, the good Professor and maybe RP depending on his mood. I guess the other 6.92 billion will fall like dominoes any day now.

 

[humber]

So far there's you, the good Professor and maybe RP depending on his mood. I guess the other 6.92 billion will fall like dominoes any day now.

As if. Spork and the cart, are totally ignored by all legitimate science.
There will be more exposure once NALSA make their position clear.

Edited by LashL: 

Removed inappropriate content.

 

[humber]

I disagree here. What happened to F = ma?

Because the Earth is massive and not a rigid body. To change the spin of the planet, you need to increase its KE.
When a force is applied to an object, that object's momentum changes and so does KE.
F=ma
F=(dp/dt) where p is momentum, and dp/dt implies a change in momentum with respect to a change in time.
Momentum, p, is related to kinetic energy, KE, by the equation KE=p^2/2m.

KE = 1/2 mv^2

p = mv

KE = 1/2 mv^2 =
1/2m (mv)^2 =
(p^2)/2m

A car has no hope of accelerating the planet because its mass is
approximately 6×10^24kg
The second reason is that the Earth is not rigid, and energy applied to it will be dissipated as work in the soil,or elsewhere.
Perhaps you can move something across a table by tapping it with a ruler. Try hitting jelly on a plate, and it will wobble, but not move as a whole.


ETA:
The point to note, Dodge69, is that whatever effects there may be, they will be negligible. It will not be possible to measure them, and certainly not with the crude equipment so far employed.
The cart is nothing that needs that attention, save to excuse it from analysis.
F1 pays not attention to the motion of the planet as a result of their powerful cars, and neither does the Olympic Committee.

 

[A.T.]

On the question of the change (or not) in the earth's momentum, there are two distinctions that must be made: that between a closed system and an open system, and that between an inertial reference frame and a noninertial reference frame.

Conservation of momentum does not hold in an open system, even with respect to an inertial reference frame, and it does not hold with respect to a noninertial reference frame, even in a closed system.

profpangloss wants to use a reference frame in which the earth is, and remains, stationary, and he wants to consider this reference frame to be practically inertial, that is, he wants to ignore the very small change in the earth's velocity. We can do this, provided we consider the earth to be external to our system. So our system consists of the cart and the air alone, but not the earth. I think this also accords with profpangloss's intuition. So far, so good. But we cannot ignore the fact that the earth exerts a force on the cart. This force acts on our system from outside it. Therefore, our system is not closed, and the momentum of our system is not conserved, but rather changes in response to the external force on it.

On the other hand, if we want momentum to be conserved, we need to consider a closed system, i.e., a system that is free from external forces. We want the cart to be part of our system, and the cart experiences a force from the earth, so the earth must be part of our system too. If we include the earth in our system, along with the cart and air, the system is closed and momentum is conserved. But, if we do things this way, we cannot treat the earth as absolutely stationary. Rather, we must choose a reference frame which is more nearly inertial than even the earth, and allow the momentum of the earth to change, with respect to this reference frame, in response to the force of the cart on the earth.

The usual way to deal with this, is to use instantaneous quantities like power (instead of work done over a certain time period). This allows you to use instantaneous inertial rest frames of accelerated objects, where the conservation laws hold.

 

[humber]

The usual way to deal with this, is to use instantaneous quantities like power (instead of work done over a certain time period). This allows you to use instantaneous inertial rest frames of accelerated objects, where the conservation laws hold.

And where there can be no acceleration because it must be static....

 

[fttw]

duplicate post

 

[fttw]

I'll be happy to do that, in detail, but I don't want to go to all the trouble if the basic premise of using the ground as the stationary reference for an inertial frame is going to be rejected out of hand, making all the mathematical work a waste of my time. That is why I need to get this argument settled first.

As I said, in the case where the earth has infinite mass, the earth forms an inertial frame. To the extent that the real earth has a mass many orders of magnitude greater than any cart, anyone who rejects an otherwise competent analysis only because the real earth is not exactly an inertial frame is making a mistake.

I'm happy to assume that the mass of the earth is infinite, and that you're free to use it as an infinite source or sink for momentum without violating any physical laws, and that, equivalently, it forms an inertial frame.

Can I see your analysis that suggests/proves that the cart can't work?

 

[windgrins]

An easy way to handle this.

As I said, in the case where the earth has infinite mass, the earth forms an inertial frame. To the extent that the real earth has a mass many orders of magnitude greater than any cart, anyone who rejects an otherwise competent analysis only because the real earth is not exactly an inertial frame is making a mistake.

I'm happy to assume that the mass of the earth is infinite, and that you're free to use it as an infinite source or sink for momentum without violating any physical laws, and that, equivalently, it forms an inertial frame.

Can I see your analysis that suggests/proves that the cart can't work?

The analyses presented for energy transfers have been screwed up many times because assumptions are made that have not held water like COM, COE without taking into account properly what Inertial Frames and Closed systems are.

It has been suggested that a simple analysis will answer the question which is a force analysis at WS.

At WS, the forces acting on the cart are the static thrust from the prop in still air. This can be derived from knowing the cartspeed=WS for an agreed upon speed-say 15mph or 30 kph depending upon preferred units.

Since the body of the cart is at WS, there are no aerodynamic forces acting on the body of the cart so this force is 0.

There is a reaction force at the wheels which is proportional to the torque on the prop and frictional losses in cart bearings and this is a function of speed.

If one computes these forces and finds that there is an unbalanced force in the direction of motion, the cart will accelerate-proving the assertion.

If one computes these forces and finds that there is an unbalanced force in the anti-direction of motion, the cart will slow-proving the opposite assertion.

This would seem to be a far easier calculation than the energy calculations just to prove the point. The more complex calculation can tell acceleration profiles given a WS or other items but this is window dressing vs showing the fundamental assertion that the cart can exceed WS mathematically.

The only reason (it seems) that we are going down this energy calculation path is that the simpler analysis yielded an answer that was different from what some of the opponents wanted to see so they resorted to a more complex analysis to see if they could find holes. Obviously if the force calculations are done correctly, any energy calculation done correctly will have to be consistent.

The impression so far is that any analysis that says the cart can work will be unacceptable to the H & H team because they have their heart set on "the cart can't work." no matter what. This is also why they can't live with the "demonstration for dummies" of the treadmill as this is a clear presentation of the idea that the cart accelerates from WS. No matter how much this is argued, armchair philosophers will not accept the results because he has mentally invested too much in his "cart can't work" theory to ever give it up.

Feel free to compute away on any of these scenarios as they are educational but don't be surprised when a "correct computation" says the cart can work, that the H & H team will have nothing of it since, in spite of all the countermanding evidence, in their minds, "the cart can't work."

I, for one, appreciate all the effort, as it is educational, but don't get any hopes up that rationality will shine through to all the posters. This isn't about that anymore (if it ever was.)


Regards,

WindGrins

 

[jduffy77]

The analyses presented for energy transfers have been screwed up many times because assumptions are made that have not held water like COM, COE without taking into account properly what Inertial Frames and Closed systems are.

It has been suggested that a simple analysis will answer the question which is a force analysis at WS.

At WS, the forces acting on the cart are the static thrust from the prop in still air. This can be derived from knowing the cartspeed=WS for an agreed upon speed-say 15mph or 30 kph depending upon preferred units.

Since the body of the cart is at WS, there are no aerodynamic forces acting on the body of the cart so this force is 0.

There is a reaction force at the wheels which is proportional to the torque on the prop and frictional losses in cart bearings and this is a function of speed.

If one computes these forces and finds that there is an unbalanced force in the direction of motion, the cart will accelerate-proving the assertion.

If one computes these forces and finds that there is an unbalanced force in the anti-direction of motion, the cart will slow-proving the opposite assertion.

This would seem to be a far easier calculation than the energy calculations just to prove the point. The more complex calculation can tell acceleration profiles given a WS or other items but this is window dressing vs showing the fundamental assertion that the cart can exceed WS mathematically.

The only reason (it seems) that we are going down this energy calculation path is that the simpler analysis yielded an answer that was different from what some of the opponents wanted to see so they resorted to a more complex analysis to see if they could find holes. Obviously if the force calculations are done correctly, any energy calculation done correctly will have to be consistent.

The impression so far is that any analysis that says the cart can work will be unacceptable to the H & H team because they have their heart set on "the cart can't work." no matter what.

Regards,

WindGrins

As I have been saying, what is so funny about HcrisPrangGlossYev is that it was, as you say, his correct force calculations which showed that the cart must work which led him to start making the energy calcs. He is less comfortable than Humber in introducing completely silly concepts to support his preconceived requirement that the cart doesn't work. At this point I, like many others, expect him to take a hiatus soon and come back later under a new name, as he is currently being forced to make more and more ridiculous claims to support his mistakes.

BTW, many, many (100+) correct calculations have been done which correctly compute both the energy and force balance.

 

[windgrins]

Watching the wheels go round and round...

As I have been saying, what is so funny about HcrisPrangGlossYev is that it was, as you say, his correct force calculations which showed that the cart must work which led him to start making the energy calcs. He is less comfortable than Humber in introducing completely silly concepts to support his preconceived requirement that the cart doesn't work. At this point I, like many others, expect him to take a hiatus soon and come back later under a new name, as he is currently being forced to make more and more ridiculous claims to support his mistakes.

BTW, many, many (100+) correct calculations have been done which correctly compute both the energy and force balance.

Yep. Totally agree. Every now and then, its just worth putting a little perspective on this whole thing. RP sort of likes watching the arguments spin forever and I sort of like resolution. I have no problem with RP's view on this however. I'm still learning so its still fun to watch.

WindGrins

 

[LashL]

This thread is suffering from a serious lack of civility. Please stop personalizing the discussion and please stop the sockpuppet allegations in-thread. I would also suggest that participants review the Membership Agreement and then be sure to abide by it or infractions are likely to start coming in faster than the wind.

Replying to this modbox in thread will be off topic  Posted By: LashL

 

[humber]

The analyses presented for energy transfers have been screwed up many times because assumptions are made that have not held water like COM, COE without taking into account properly what Inertial Frames and Closed systems are.

It is typical of woo and pseudo-science to whine about form, and the minutia of definitions.

You do not have the necessary data: tyres, cart weight, air mass, humidity, chassis stiffness, ground surface, wind shear, gradient, temperature....to worry about the inconsequential differences produced by the above lament.

But quite happy to accept a treadmill from Walmart as a "frame of reference"

It has been suggested that a simple analysis will answer the question which is a force analysis at WS.

At WS, the forces acting on the cart are the static thrust from the prop in still air. This can be derived from knowing the cartspeed=WS for an agreed upon speed-say 15mph or 30 kph depending upon preferred units.

While strenuously avoiding the matter of fluid dynamics.

The only reason (it seems) that we are going down this energy calculation path is that the simpler analysis yielded an answer that was different from what some of the opponents wanted to see...
Feel free to compute away on any of these scenarios as they are educational but don't be surprised when a "correct computation" says the cart can work, that the H & H team will have nothing of it since, in spite of all the countermanding evidence, in their minds, "the cart can't work."

You mean after how many versions and ad-hoc changes?
The trial data does not match the theory in the submission report, not any other, and completely contradicts the Javaprop data that Spork hopes will be forgotten about.
I doubt you will win this one.