Merged DDWFTTW in a “Real” Wind

[humber]

Humber,

In all seriousness, I suggest you ask someone about my previous two posts who has had or teaches a high school senior or first level college physics course. No need to report back to us. Just do it for your own benefit. There are plenty of other forums on the internet where you can get this kind of question answered from a neutral party. Just quote the actual text and present it for an answer. Again, no need to report back to us.


Regards and signing off for the evening.

WindGrins

Answer to your claim by addressing the balloon and bird and the tablecloth.

 

[humber]

Really? Good to know, thanks!

You forgot your relative motion and made downwind absolute, so things never come the other way.

The problem is that a closing speed of 1200mph, can't be mimicked by having one object stationary and the other moving at 600mph. Want to make that bird travel at 1200mph?

I doubt that the total KE of the plane and the bird will be the same, as when both are traveling at 600mph but in opposite directions.

 

[CORed]

So, a bird traveling at 1mph relative to the plane, but the other way.
Can then be simulated by turning the fan off, and throwing the bird at the nose of the plane at 1mph? Or is there a difference between closing speed and relative speed? Won't the total KE be different in the air, than when the fan is off, even if they were traveling the same way?

I can't make any sense of this question whatsoever.

I perform the following experiment. With a table, table cloth and some dinnerware. Using a strain gauge to measure the force as I pull the cloth at speed x, I note a moderate force of 20N. That is due to the force of friction between the table cloth and the table. Using the same strain gauge, I tether the tablecloth to the wall, and pull the table at speed x. The strain gauge still reads 20N, but now there is a static force of 20N in the wall.
I also note that I am doing a lot more work, and that there is noise that was not there before. There's more to motion than velocity. The TM is a triple joke.

Congratulations humber. You just proved that it takes more force and more work to move a table than it does to move a tablecloth.

Now what was it that you thought your were proving?

 

[windgrins]

CORed states the case more concisely than WG!

I can't make any sense of this question whatsoever.

Congratulations humber. You just proved that it takes more force and more work to move a table than it does to move a tablecloth.

Nicely and concisely stated. Agreed.

WindGrins:

 

[mender]

You forgot your relative motion and made downwind absolute, so things never come the other way.

The problem is that a closing speed of 1200mph, can't be mimicked by having one object stationary and the other moving at 600mph. Want to make that bird travel at 1200mph?

I doubt that the total KE of the plane and the bird will be the same, as when both are traveling at 600mph but in opposite directions.

If there is a closing speed of 1200 mph, any combination of speeds that add up to 1200 will produce exactly the same effect on the objects when the objects collide. 600 + 600, 0 + 1200, 376.9 + 823.1, etc; every one is the same to the objects, the only thing changing is the speed of the observer relative to the objects and since the observer isn't involved, that can't affect what happens to the objects.

Are you disputing that?

 

[humber]

I can't make any sense of this question whatsoever.

Congratulations humber. You just proved that it takes more force and more work to move a table than it does to move a tablecloth.

Now what was it that you thought your were proving?

For example, a cart at windspeed is not the same as one held in place by static force to the air (thrust) as the belt moves beneath it.
The prop does provide static thrust, so the air may as well be equated to the wall, and the belt to the table, where the motor does work on the belt, but not the cart. When pulling the table cloth, you do work on the dinner ware ( not just the friction), but when pulling the table, you do work only on the table and friction to the table, but none on the dinnerware.
The TM is a way of dealing with the forces, so it stays in place.

 

[mender]

For example, a cart at windspeed is not the same as one held in place by static force to the air (thrust) as the belt moves beneath it.
The prop does provide static thrust, so the air may as well be equated to the wall, and the belt to the table, where the motor does work on the belt, but not the cart. When pulling the table cloth, you do work on the dinner ware ( not just the friction), but when pulling the table, you do work only on the table and friction to the table, but none on the dinnerware.
The TM is a way of dealing with the forces, so it stays in place.

The tablecloth and the strain gauge don't care what's causing the relative motion between the table and the cloth, and the cloth and strain gauge react exactly the same regardless.

The cart doesn't care what's causing the relative motion between the air and the surface under its wheels, and the cart reacts exactly the same regardless.

You can use an aircraft carrier or a wind tunnel or a treadmill to provide the relative motion of the air and the surface under the cart. Some methods are obviously more energy intensive than other, but it makes no difference to the cart how the relative motion is generated, only that it is generated.

You're getting work mixed into this now; don't, it'll only confuse you more. Stick with the forces.

 

[windgrins]

RP-Haven't heard from you lately

RP,

I think I can remain polite but I think I'm getting sleepy. I don't know if you have been following but do you think this thread is illustrating any deep subtle ideas at this point?

I'm having fun but I think I have arrived at the following:

Reductio ad absurdum (Latin: "reduction to the absurd") is a form of argument in which a proposition is disproven by following its implications logically to an absurd consequence.[1

It is questionable whether any of us are sane anymore!

Regards,

WindGrins

 

[mender]

Yeah, I'm done for tonight as well.

 

[humber]

If there is a closing speed of 1200 mph, any combination of speeds that add up to 1200 will produce exactly the same effect on the objects when the objects collide. 600 + 600, 0 + 1200, 376.9 + 823.1, etc; every one is the same to the objects, the only thing changing is the speed of the observer relative to the objects and since the observer isn't involved, that can't affect what happens to the objects.

Are you disputing that?

When traveling at 10mph, and and object behind is traveling at 5mph, it is not traveling backwards, as you often say it is, but 5 mph slower forwards.
To say the former, is to confuse velocity and differential speed.
Backwards is quite different. You have taken the speed differential of car and ground, to mean that the road is moving the other way.
Your error shows up in my example of the 1mph bird.

KE of missile at 600mph = 10000J
KE of second missile at 600mph = 10000J
total 20000J
KE of one but same missile at 1200mph = 40000J.

 

[humber]

The tablecloth and the strain gauge don't care what's causing the relative motion between the table and the cloth, and the cloth and strain gauge react exactly the same regardless.

Free table cloth, puller does work on dinner ware, so moves it.
Tethered table cloth, puller does work on table, so moves it. No work is done on the dinner ware by anyone or anything.

Static force tethers the cart to the air, as the motor does work on the belt, and some friction related work on the cart. The cart does not move. The rest is a bit of mechanics to pull that off.

ETA:
It's a howling error, Mender. A car travels at 10mph. I could model that by putting a toy car on top of a book, and moving the car 5mph one way, and the book 5mph the other.
When it comes to making a model where one is stopped, I can stop the book and move the car at 10mph, or stop the car and move the book at 10mph the other way. Confusing differential and relative speeds.
If I spin some BS about the still air of a room, I may convince you that the still object is traveling at the speed of the wind.

 

[mender]

When traveling at 10mph, and and object behind is traveling at 5mph, it is not traveling backwards, as you often say it is, but 5 mph slower forwards.
To say the former, is to confuse velocity and differential speed.
Backwards is quite different. You have taken the speed differential of car and ground, to mean that the road is moving the other way.
Your error shows up in my example of the 1mph bird.

KE of missile at 600mph = 10000J
KE of second missile at 600mph = 10000J
total 20000J
KE of one but same missile at 1200mph = 40000J.

KE is relative.

 

[mender]

A car travels at 10mph. I could model that by putting a toy car on top of a book, and moving the car 5mph one way, and the book 5mph the other.
When it comes to making a model where one is stopped, I can stop the book and move the car at 10mph, or stop the car and move the book at 10mph the other way. Confusing differential and relative speeds.

To the car, all conditions are exactly the same. That's what FOR is all about; the forces are identical and that's what the objects react to.

If I spin some BS about the still air of a room, I may convince you that the still object is traveling at the speed of the wind.

And you'd be correct - but can you convince yourself?

 

[recursive prophet]

It is questionable whether any of us are sane anymore!

We are all here..because we are not all there. That's what it says on the licence plates in Cartville. And I'm always here windgrins, but lurking is my default.

I must say I've been thoroughly enjoying your entry into our warped little community, and mender is always fun to read. He knows Humber is right about almost everything, but just loves to argue with him anyway.

 

[zooterkin]

Humber is right about almost everything

 

[spork]

There is no difference between closing speed and relative speed. They are the same concept.

In one-dimension this is true; but in 2 or more dimensions it's not always true. Consider the case of two cars passing on the highway - one northbound and the other southbound. They pass with several car widths between them. Let's say each is going 60 mph. At the moment they're adjacent to one another, their relative speed is 120 mph. But their closing speed is 0 mph at that moment.

During their approach or departure, their relative speed and closing speed become more closely aligned the further away from this point at which they're adjacent.

DISCLAIMER: I don't read much of what humber posts, so it's possible your answer to him was right on the money in the given context. I just felt the brief need to converse with someone that can... communicate.

 

[humber]

KE is relative.

Sloganism.

To the car, all conditions are exactly the same. That's what FOR is all about; the forces are identical and that's what the objects react to.

Well, a car travels over the ground, but if you like, you can say the ground is still and the car moves, or the car is still and the road moves. The TM is the latter.
The conditions are by no means the same, and everything from the tyre pattern to the work done is totally different.

It is directly analogous to the tethered table cloth. When the cloth moves the tableware moves and so travel across the table. When the table cloth is tethered, static force from the wall holds it in place as the table moves.
Not at all the same thing. You all say the prop produces static thrust and it does, so like the tethered table cloth and dinner ware is held there by static force.
Playing FOR my "mentally" holding one object still and "imagineering" the other moving is not the same as actually doing that.
The cart is in a latched state, and does not move relative to the belt. The motion that is seen, is motion relative to the ground, or more precisely the supporting plate under the belt.

 

[humber]

In one-dimension this is true; but in 2 or more dimensions it's not always true. Consider the case of two cars passing on the highway - one northbound and the other southbound. They pass with several car widths between them. Let's say each is going 60 mph. At the moment they're adjacent to one another, their relative speed is 120 mph. But their closing speed is 0 mph at that moment.

No. The speed of the cars remains the same; 60 mph each way, so a differential speed, not relative speed of 120mph. If they were 1km apart, there would be no doubt that the difference was 120mph, at 100m the same, and at 1m and so on. Closing speed applies when the cars are traveling in the same direction.
ETA:
"Closing speed" is often used for closing distance: A car traveling at 20mph, is 10mph faster than one traveling at 10mph, regardless of distance or when passing or not.
Hollywood aside, closing speed refers to collisions, and is:
"The magnitude of the velocity difference at impact is called the closing speed."
The definition, and confusion with differential velocity or speed, has been the subject of a few forum squabbles.
A car traveling in the same direction as an other, may approach and then maintain the same speed for as long as it may, but that can't happen with two cars traveling in opposite directions.

Velocity is relative, so direction is too.

DISCLAIMER: I don't read much of what humber posts, so it's possible your answer to him was right on the money in the given context. I just felt the brief need to converse with someone that can... communicate.

I know you won't respond to my balloon challenge, and you can bluff that out. But, it's a consolation prize, because when directly asked, none of the Carteers were willing to back you, nor did any volunteer to do so.

 

[EHocking]

<snip obfuscation to cover up backpedalling>

You contradicted yourself on balloons running at WS and now are attempting a back pedal when this was pointed out.

Again, the relevant posts:

Originally Posted by humber ...analysis and physical explanations that say the balloon will not reach WS.

Originally Posted by humber ...a helium balloon ...moves at the speed of the wind in the direction of the wind. This is what makes ballooning so fascinating: there is no wind blowing on you and no wind noise.

 

[windgrins]

Sporks comment about closing velocity

In one-dimension this is true; but in 2 or more dimensions it's not always true. Consider the case of two cars passing on the highway - one northbound and the other southbound. They pass with several car widths between them. Let's say each is going 60 mph. At the moment they're adjacent to one another, their relative speed is 120 mph. But their closing speed is 0 mph at that moment.

During their approach or departure, their relative speed and closing speed become more closely aligned the further away from this point at which they're adjacent.

DISCLAIMER: I don't read much of what humber posts, so it's possible your answer to him was right on the money in the given context. I just felt the brief need to converse with someone that can... communicate.

Spork,

You are clearly correct sir. My statement only stands correct if the objects are moving along the same single dimension. Your comment is a good clarification as it lends insight. I believe this is even why the idea of Velocity Made Good (VMG) was created. My reply was in to a Humber question (sort of a question):

So, a bird traveling at 1mph relative to the plane, but the other way.
Can then be simulated by turning the fan off, and throwing the bird at the nose of the plane at 1mph? Or is there a difference between closing speed and relative speed? Won't the total KE be different in the air, than when the fan is off, even if they were traveling the same way?

So I inferred from this question a "collision course" of bird and plane but some of us had a problem understanding what was being asked.

You're comment adds additional insight. Thanks,

Thanks Again,

WindGrins